Severe Combined Immunodeficiency (SCID) is a group of inherited disorders presenting in early infancy, characterized by profound deficiency of both T-cell and B-cell function, resulting in life-threatening susceptibility to severe infections typically within the first few months of life if untreated.

Severe Combined Immunodeficiency (SCID) — Paediatrics

Severe Combined Immunodeficiency (SCID) — breaking down the name:

"Severe" = life-threatening, fatal without treatment
"Combined" = both arms of adaptive immunity are affected (T cells and B cells)
"Immunodeficiency" = the immune system cannot mount adequate defence

SCID refers to a heterogeneous group of genetic disorders characterised by profound deficiency in both T-cell and B-cell development and/or function, resulting in absent or severely impaired adaptive immunity. ^[1]^[2]

It is the most severe form of primary immunodeficiency (PID) — often called the "paediatric immunological emergency" because it is uniformly fatal in infancy if untreated, typically within the first 1–2 years of life from overwhelming infection. ^[1]^[2]^[3]

High Yield — Defining Feature

SCID is a heterogeneous group of disorders with impaired B and T cell development. It is fatal without treatment. ^[1]^[2] The key unifying feature is absent or profoundly reduced T-cell numbers and/or function, with variable B-cell and NK-cell involvement depending on the specific genetic defect.

2. Epidemiology

Overall incidence: approximately 1 in 50,000–100,000 live births in older literature
1 in 25,000–30,000 when including all genetic subtypes detected by newborn screening ^[4]
Newborn screening (NBS) programmes using TREC assays have revealed that the true incidence is higher than previously appreciated

Male predominance because the most common single genetic cause is X-linked (IL2RG mutation, accounting for ~40–50% of all SCID cases in many populations)
Several (PID) are X-linked, accounting for male preponderance of some PID ^[5]

Typically presents in the first 3–6 months of life
Maternal transplacental IgG provides some passive protection in the first weeks, but this wanes by 3–6 months
T-cell defects manifest earlier than isolated antibody defects because T cells are critical for viral/fungal defence from birth

In Hong Kong, newborn screening for SCID is not yet universally implemented (as of 2025), though pilot programmes exist
Certain genetic variants (e.g., RAG1/RAG2 mutations) may be relatively more common in Asian populations
Consanguinity (though less common in Hong Kong than in the Middle East) increases the risk of autosomal recessive forms

3. Anatomy and Function: The Immune System Relevant to SCID

Understanding SCID requires understanding how T and B lymphocytes develop, because SCID is fundamentally a disorder of lymphocyte development.

Bone marrow HSC → Common lymphoid progenitor → migrates to THYMUS
     ↓
  In thymus: T-cell receptor (TCR) gene rearrangement
     ↓
  Positive selection (cortex) → Negative selection (medulla)
     ↓
  Mature naïve T cells (CD4+ helper, CD8+ cytotoxic) → enter circulation

TRECs (T-cell Receptor Excision Circles): small circular DNA by-products of TCR gene rearrangement. They are a marker of recent thymic emigrants (newly produced T cells). In SCID, TRECs are very low or absent because T cells are not being produced. This is the basis of newborn screening.

Bone marrow HSC → Pro-B cell → Pre-B cell → Immature B cell → exits to periphery
     ↓
  In periphery: encounters antigen → (with T-cell help) → class switching, affinity maturation
     ↓
  Plasma cells (produce IgG, IgA, IgE) + Memory B cells

Key point: Even if B cells are present in number (T⁻B⁺ SCID), they cannot function without T-cell help (CD4⁺ T helper cells drive class-switch recombination from IgM → IgG/IgA/IgE). Therefore, all forms of SCID have functionally impaired humoral immunity regardless of B-cell count.

NK cells arise from the common lymphoid progenitor but do not require thymic maturation
Some SCID subtypes affect NK cells (e.g., X-linked SCID, JAK3 deficiency) while others spare them (e.g., RAG1/2 deficiency, IL-7Rα deficiency)
NK-cell status helps classify the subtype (T⁻B⁺NK⁻ vs T⁻B⁺NK⁺ vs T⁻B⁻NK⁺ etc.)

The thymus is a lymphoepithelial organ in the anterior mediastinum that is critical for T-cell maturation
In SCID, the thymus is hypoplastic or absent because without T-cell precursors entering or being able to proliferate, the thymus involutes
Thymus shadow is absent on CXR — this is a key radiological finding ^[1]^[2]

Why is the thymic shadow absent in SCID?

The thymus requires T-cell progenitors to populate it and undergo selection. In SCID, T-cell precursors cannot develop properly → the thymus has no lymphocytes to expand → it remains tiny/involuted → no thymic shadow on CXR. This is different from the normal neonate where the thymus is relatively large and creates a characteristic "sail sign."

4. Aetiology and Pathophysiology

SCID is caused by monogenic (single-gene) defects — there are > 20 known genetic causes, but they all converge on the same final pathway: failure of T-cell development ± B-cell and NK-cell development.

SCID — Mostly single gene disorders ^[5]

The most clinically useful classification is by which lymphocyte populations are affected:

Phenotype	T cells	B cells	NK cells	Genetic Defects	Inheritance
T⁻B⁺NK⁻	Absent	Present	Absent	IL2RG (γc chain), JAK3	XLR, AR
T⁻B⁺NK⁺	Absent	Present	Present	IL7Rα, CD3δ/ε/ζ, PTRCRA, CD45	AR
T⁻B⁻NK⁺	Absent	Absent	Present	RAG1, RAG2, DCLRE1C (Artemis), LIG4, NHEJ1	AR
T⁻B⁻NK⁻	Absent	Absent	Absent	ADA (adenosine deaminase), PNP (purine nucleoside phosphorylase), AK2 (reticular dysgenesis)	AR

High Yield — Most Common SCID Subtypes

IL2RG (X-linked, loss of function) is the single most common cause of SCID, accounting for ~40–50% of cases. ^[4] It encodes the common gamma chain (γc) shared by receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Loss of γc disrupts signalling through all these cytokines → catastrophic failure of T-cell, B-cell, and NK-cell development.

4.2 Detailed Pathophysiology of Major Subtypes

Gene: IL2RG on Xq13.1
Inheritance: X-linked recessive (XLR) → affects boys
Protein: Common gamma chain (γc) — a shared subunit of the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21
Why T cells are absent: IL-7 signalling is essential for T-cell development in the thymus. Without the γc chain, the IL-7 receptor cannot signal → T-cell progenitors fail to survive and proliferate → no T cells
Why NK cells are absent: IL-15 signalling drives NK-cell development. Without γc, IL-15 receptor cannot signal → no NK cells
Why B cells are present but non-functional: B cells develop through alternative pathways but require T-cell help for class switching → B cells are present in number but produce only low-quality IgM, with absent IgG, IgA, IgE
Phenotype: T⁻B⁺NK⁻

Gene: JAK3 (autosomal recessive)
JAK3 is the intracellular tyrosine kinase that binds to γc and transduces signals downstream
Phenotypically identical to X-linked SCID (T⁻B⁺NK⁻) but affects both sexes
This is the "autosomal recessive mimic" of X-linked SCID

Gene: IL7R (autosomal recessive)
IL-7Rα pairs with γc to form the complete IL-7 receptor
IL-7 is specifically critical for T-cell development (not NK cells)
Without IL-7Rα: T cells fail to develop, but NK cells (which use IL-15/γc) are spared
Phenotype: T⁻B⁺NK⁺

Gene: RAG1 or RAG2 (autosomal recessive)
RAG1 and RAG2 are recombination-activating genes essential for V(D)J recombination — the process by which T-cell receptors (TCR) and B-cell receptors (immunoglobulin) generate their diversity
Without RAG: neither TCR nor immunoglobulin genes can rearrange → no mature T cells, no mature B cells
NK cells do NOT require V(D)J recombination (they use germline-encoded receptors like KIR) → NK cells are spared
Phenotype: T⁻B⁻NK⁺
Hypomorphic (partial loss-of-function) RAG mutations → Omenn syndrome: a "leaky" SCID with oligoclonal, autoreactive T cells → erythroderma, lymphadenopathy, hepatosplenomegaly, eosinophilia, elevated IgE

These genes are involved in non-homologous end joining (NHEJ), the DNA repair pathway required after RAG-mediated double-strand breaks during V(D)J recombination
Same phenotype as RAG deficiency (T⁻B⁻NK⁺)
Additionally: patients may show radiation sensitivity (important if considering conditioning for HSCT)

Gene: ADA (autosomal recessive)
ADA is an enzyme in the purine salvage pathway
Without ADA: deoxyadenosine accumulates → converted to dATP → dATP is directly toxic to developing lymphocytes (inhibits ribonucleotide reductase → blocks DNA synthesis; also triggers apoptosis)
All lymphocyte lineages are affected (T, B, and NK) because all are rapidly dividing cells sensitive to purine metabolite toxicity
Phenotype: T⁻B⁻NK⁻
Unique features: costochondral junction abnormalities ("cupping and flaring" on CXR), skeletal abnormalities, may have neurological features
Accounts for ~15% of all SCID

Gene: AK2 (adenylate kinase 2)
The most severe form of SCID — affects not only lymphocytes but also myeloid cells (neutrophils)
Results in combined immunodeficiency + severe congenital agranulocytosis + sensorineural deafness
Extremely rare

5. Classification

As per the table in Section 4.1 above — T⁻B⁺NK⁻, T⁻B⁺NK⁺, T⁻B⁻NK⁺, T⁻B⁻NK⁻

SCID falls under Category 1: Immunodeficiencies affecting cellular and humoral immunity within the broader classification of Inborn Errors of Immunity.

Primary immunodeficiencies / inborn errors of immunity (10 types) ^[4]

The 10 categories are:

Combined immunodeficiencies (CID) — SCID falls here ^[4]
CID with associated or syndromic features (e.g., Wiskott-Aldrich, DiGeorge, Hyper-IgE)
Predominantly antibody deficiencies (e.g., XLA, CVID)
Diseases of immune dysregulation
Congenital defects of phagocyte number/function (e.g., CGD)
Defects in intrinsic and innate immunity
Autoinflammatory disorders
Complement deficiencies
Bone marrow failure
Phenocopies of PID

Pattern	Examples
X-linked recessive	IL2RG (most common single cause)
Autosomal recessive	JAK3, IL-7Rα, RAG1/2, ADA, Artemis, AK2, PNP

Typical SCID: Complete absence of T cells, severe phenotype, presents early in infancy
Leaky/Atypical SCID: Hypomorphic mutations allowing some residual T-cell development → partial immunity → later or atypical presentation
- Omenn syndrome: Oligoclonal T-cell expansion → erythroderma, lymphadenopathy, eosinophilia, high IgE (caused by hypomorphic RAG1/2 or other mutations)

6. Clinical Features

SCID usually presents with failure to thrive (FTT), recurrent severe infections, and chronic diarrhoea ^[1]^[2]

The typical SCID infant appears well at birth (due to maternal IgG and sterile intrauterine environment) but then develops progressive, severe, recurrent infections beginning at 2–6 months of age as maternal antibodies wane and environmental pathogen exposure increases.

6.2 Symptoms (with Pathophysiological Basis)

Symptom	Pathophysiology	Typical Pathogens
Persistent oral thrush (candidiasis)	Absent T-cell–mediated immunity against fungi; Th17 responses are critical for mucocutaneous candidal defence	Candida albicans
Severe pneumonia / bronchiolitis	Absent T-cell viral clearance + absent functional antibodies → cannot control respiratory viruses	RSV, parainfluenza, adenovirus, Pneumocystis jirovecii
PJP (Pneumocystis jirovecii pneumonia)	PJP requires CD4⁺ T-cell–mediated clearance; classic opportunistic infection of SCID	P. jirovecii
Chronic / protracted diarrhoea	T-cell deficiency → cannot clear enteric viruses or parasites; mucosal immune barrier is deficient → chronic infection and villous atrophy → malabsorption	Rotavirus, norovirus, Cryptosporidium, Giardia, adenovirus
Disseminated viral infections	No cytotoxic T cells (CD8⁺) to kill virus-infected cells → unchecked viral replication and dissemination	CMV, EBV, adenovirus, HSV, VZV
Disseminated BCG infection (BCGitis/BCGosis)	Live attenuated BCG vaccine given at birth (routine in HK) → in SCID infants, cannot mount T-cell response to control the attenuated mycobacterium → disseminated mycobacterial disease	Mycobacterium bovis BCG
Recurrent skin infections / abscesses	Poor cellular immunity → impaired neutrophil/macrophage activation (which requires T-cell cytokines like IFN-γ)	S. aureus, fungi
Sepsis / meningitis	Global immune failure → inability to contain bacteraemia	Various bacteria, fungi

BCG Vaccine — Critical Paediatric Point for Hong Kong

In Hong Kong, BCG is given at birth as part of the childhood immunisation programme. In a SCID infant, BCG can cause disseminated BCGosis — a life-threatening complication. This is why early identification of SCID (ideally through newborn screening before BCG vaccination) is crucial. BCG vaccine will disseminate in SCID. ^[4] Complications from a live vaccine is a warning sign for PID. ^[2]

Susceptible pathogens in SCID: All — Fungus (PJP), BCG vaccine will disseminate, Candida, viruses ^[4]

Mechanism: Chronic infections (especially chronic diarrhoea) → malabsorption → caloric deficit → poor weight gain
Also: chronic immune activation → catabolic state → wasting
FTT is a classical presenting feature ^[1]^[2]

Mechanism: As above — enteric viral infections (rotavirus, norovirus), parasites (Cryptosporidium), or bacterial overgrowth → villous atrophy, malabsorption
May be intractable and contribute significantly to FTT

Persistent eczema-like rash or erythroderma: may indicate Omenn syndrome (oligoclonal autoreactive T cells causing inflammatory skin disease) or graft-versus-host disease (GvHD) from transplacentally acquired maternal T cells that attack the infant's tissues
Persistent nappy rash / candidal dermatitis: mucocutaneous candidiasis

6.3 Signs (with Pathophysiological Basis)

Sign	Pathophysiology
Absent thymic shadow on CXR	Thymus is hypoplastic/absent due to lack of T-cell precursors; no thymic expansion ^[1]^[2]
Absent or hypoplastic tonsils	Tonsils are secondary lymphoid organs populated by T and B cells; in SCID, lymphocytes are absent or severely reduced → tonsils do not develop
Absent peripheral lymph nodes	Same mechanism — lymph nodes fail to populate without functional lymphocytes

Clinical Pearl — CXR in SCID

Always look at the CXR of any infant with recurrent infections. An absent thymic shadow in a neonate/infant is a red flag for SCID. The normal neonatal thymus is large and produces a "sail sign" or widened mediastinum. Its absence is pathological and warrants urgent lymphocyte subset analysis.

Weight below 3rd centile / crossing centiles downward (FTT)
Length/height may also be affected if chronic disease persists
Wasting and muscle loss

Sign	Significance
Oral thrush that is persistent and refractory to topical antifungals	Suggests T-cell deficiency (normal infants may have transient thrush, but it responds to treatment)
Respiratory distress / tachypnoea / crackles	PJP pneumonia (bilateral diffuse infiltrates on CXR), viral pneumonitis, or bacterial pneumonia
Hepatosplenomegaly	Disseminated infection (CMV, BCG), GvHD from maternal T cells, or Omenn syndrome
Skin erythroderma / desquamation	Omenn syndrome or maternal T-cell engraftment (GvHD)
BCG scar ulceration / regional lymphadenopathy / disseminated BCG	BCGitis or BCGosis — look for enlarged axillary lymph node, non-healing ulcer at BCG site
Perianal disease	Chronic infection / abscess

Low absolute lymphocyte count (ALC) ^[1]^[2]
Lymphopenia in infants is defined by < 2,500 cells/µL ^[2]
In older children (> 5 years): < 1,500 cells/µL ^[2]
Persistent lymphopenia is a warning sign for PID ^[2]

High Yield — Why is ALC important?

A normal neonate has physiological lymphocytosis (ALC typically 4,000–10,000/µL). An ALC < 2,500/µL in an infant is abnormal and should raise immediate suspicion for SCID. This is often the first laboratory clue picked up on a routine CBC before specific immunological testing is done.

Features suggestive of primary immunodeficiency (PID): ^[2]

Patient Factors:

Family history of immunodeficiency or unexplained early death (before 30)
Failure to gain weight or grow normally (failure to thrive)
Persistent lymphopenia (< 1,500 cells/µL in > 5 years; < 2,500 cells/µL in younger children)

Infection-related Factors:

≥ 6 new infections within 1 year
≥ 2 serious sinus infections or pneumonias within 1 year
≥ 4 new ear infections within 1 year
≥ 2 episodes of sepsis or meningitis in a lifetime
Recurrent or resistant candidiasis
Recurrent tissue or organ abscesses
Infection with an opportunistic organism
Unexplained autoimmunity or fever
Chronic diarrhoea
Granuloma
Non-healing wounds
Extensive skin lesions

Treatment-related Factors:

≥ 2 months of antibiotics with little effect
Need for IV antibiotics or hospitalisation to clear infections
Complications from a live vaccine ^[2]

A brief outline of key investigations for orientation:

Test	Purpose	Expected in SCID
CBC with differential	ALC	Low ALC (< 2,500/µL in infants) ^[1]^[2]
Lymphocyte subsets (flow cytometry)	T, B, NK cell counts	Low / zero T-cell count ^[4]
Immunoglobulins (IgG, IgA, IgM, IgE)	Humoral function	Low IgG (once maternal IgG wanes), low/absent IgA, IgE; IgM variable
TREC (T-cell Receptor Excision Circles) on newborn screening	Thymic output	Low TREC on newborn screening ^[4] — this is the basis for NBS
CXR	Thymic shadow	Absent thymus shadow ^[1]^[2]
Genetic testing	Confirm specific gene defect	Targeted panels or whole exome/genome sequencing
Naïve T cells (CD45RA⁺)	Distinguish true SCID from maternal T-cell engraftment	Low naïve T cells ^[4]
Mitogen proliferation assays	T-cell function	Absent or severely reduced proliferative response to PHA, ConA

Feature	SCID	DiGeorge Syndrome	XLA	CVID
T cells	↓↓↓ or absent	↓ (variable)	Normal	Normal or mildly ↓
B cells	Variable (depends on subtype)	Normal	↓↓↓ or absent	Normal number, ↓ function
NK cells	Variable	Normal	Normal	Normal
Immunoglobulins	All ↓	Variable	Pan-hypogammaglobulinaemia	↓↓↓ IgG, ↓ IgA/IgE
Thymic shadow	Absent	Absent/hypoplastic	Normal	Normal
Inheritance	XLR or AR	AD (22q11.2 deletion)	XLR (BTK)	Various
Age of onset	< 6 months	Birth (cardiac/hypocalcaemia)	4–6 months	Childhood to young adult

9. Specific Paediatric Considerations

Neonates and infants have physiological lymphocytosis — a "normal" CBC in a newborn shows lymphocyte predominance with ALC > 4,000/µL
Therefore, an ALC that appears "normal" for an adult (e.g., 1,800/µL) is abnormal and concerning in an infant
Always use paediatric reference ranges

Transplacental IgG transfer peaks in the third trimester → term neonates have IgG levels ≈ maternal levels
This passively protects against bacterial infections initially but does NOT compensate for absent T-cell immunity (hence early presentation with viral/fungal infections)
Maternal IgG has a half-life of ~21 days → by 3–6 months, levels decline → antibody-mediated infections also begin

Parents must be counselled about the emergency nature of the diagnosis
Isolation precautions: avoid daycare, sick contacts, and public spaces until definitive treatment
No live vaccines should be given (BCG, rotavirus, OPV, MMR, varicella) — and live vaccines should be withheld from household contacts (e.g., no OPV to siblings)
Irradiated and CMV-negative blood products only — to prevent transfusion-associated GvHD and CMV transmission
Breastfeeding is generally safe and encouraged (maternal IgA provides mucosal protection) — unless mother is CMV-positive (discuss risks)
Genetic counselling for family (X-linked recessive: carrier mother, 50% of sons affected)

Parents provide consent for all investigations and treatments as the patient is an infant
For HSCT (bone marrow transplant), detailed informed consent covering risks of conditioning, GvHD, and long-term complications is essential

Treatment of SCID: HSCT (haematopoietic stem cell transplantation) ^[4]^[5]

Definitive treatment: HSCT — this is the only curative treatment for most forms of SCID
- Best outcomes when performed before 3.5 months of age and before onset of infections
- HLA-matched sibling donor is ideal; haploidentical (parent) or matched unrelated donor (MUD) also used
- Survival > 90% if transplanted early without active infection
Gene therapy: Available/emerging for specific subtypes (ADA-SCID, X-linked SCID) — involves inserting a functional copy of the defective gene into the patient's own haematopoietic stem cells
Enzyme replacement therapy (ERT): PEG-ADA (pegademase bovine or elapegademase) for ADA-SCID as a bridge to HSCT or gene therapy
Supportive care: Antimicrobial prophylaxis (cotrimoxazole for PJP, antifungals, antivirals), IVIG replacement, nutritional support, strict infection control

High Yield Summary

SCID is a heterogeneous group of disorders with impaired B and T cell development — it is fatal without treatment ^[1]^[2]
Most common cause: IL2RG (X-linked, loss of function) — incidence ~1 in 25,000–30,000 ^[4]
Typical presentation: Infant < 6 months with FTT, recurrent severe infections, chronic diarrhoea ^[1]^[2]
Susceptible pathogens: All — fungus (PJP), BCG vaccine will disseminate, Candida, viruses ^[4]
Key investigations: Low ALC (< 2,500/µL in infants) ^[1]^[2], absent thymus shadow on CXR ^[1]^[2], low TREC on newborn screening ^[4], low/zero T-cell count ^[4]
Definitive treatment: HSCT (haematopoietic stem cell transplantation) ^[4]^[5] — best outcomes if done early, before infections
Several PIDs are X-linked → male preponderance ^[5]
Delayed treatment is associated with complications such as bronchiectasis ^[5]
Lymphopenia in infants: < 2,500 cells/µL is abnormal — always use paediatric reference ranges ^[2]
Avoid all live vaccines in SCID — especially BCG (given at birth in HK)

Active Recall - SCID (Definition, Epidemiology, Pathophysiology & Clinical Features)

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.410, Primary Immunodeficiency — Combined Deficiency / SCID section) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.638, SCID and Clinical Manifestations of PID) ^[3] Senior notes: AOS - Paeds.pdf (Question 9 and related PID discussion) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (Combined immunodeficiencies section, Block A and Block C summary) ^[5] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.9, Clinical Utility of Molecular Diagnostic Testing for PID) ^[6] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf / Block C - A child with recurrent infections_ Primary immunodeficiencies.pdf (IUIS classification, SCID subtypes)

When an infant presents with the classic triad of failure to thrive, recurrent severe infections, and chronic diarrhoea ^[1]^[2], the differential diagnosis is not just "SCID versus everything else." The real clinical challenge is threefold:

Is this a primary immunodeficiency (PID) at all, or a secondary cause of immunodeficiency?
If it is a PID, which category of PID is it? (SCID vs. other PIDs that may mimic SCID)
If it is SCID, which specific genetic subtype? (important for treatment planning)

We will work through each level systematically.

The first step is to decide whether this infant genuinely has an immune defect or whether the recurrent infections have another explanation. Most children with recurrent infections are immunologically normal — they simply have high environmental exposure (e.g., daycare, older siblings). The warning signs that push you toward a PID workup are those from the Jeffrey Modell Foundation / IUIS criteria:

Features suggestive of primary immunodeficiency (PID) ^[2]:

Family history of immunodeficiency or unexplained early death (before 30)
Failure to gain weight or grow normally (FTT)
Persistent lymphopenia
≥ 6 new infections within 1 year
≥ 2 serious sinus infections or pneumonias within 1 year
≥ 2 episodes of sepsis or meningitis in a lifetime
Recurrent or resistant candidiasis
Infection with an opportunistic organism
Complications from a live vaccine
≥ 2 months of antibiotics with little effect

Clinical Pearl — When to Suspect PID Over Normal Childhood Infections

A healthy child in daycare may have 6–8 URTIs per year — but these are mild, self-limiting, and caused by common viruses. PID should be suspected when infections are severe (requiring hospitalisation/IV antibiotics), caused by unusual organisms (PJP, Candida deep tissue, atypical mycobacteria), or fail to respond to standard treatment. Opportunistic infections in an infant are PID until proven otherwise.

2. Differential Diagnosis of SCID Specifically

2.1 Other Primary Immunodeficiencies (PIDs) That May Mimic SCID

This is the most important differential — distinguishing SCID from other PIDs that present in early infancy with severe infections. The key discriminating features are the lymphocyte phenotype (T/B/NK cell counts), age at onset, type of infections, and associated syndromic features.

These fall under IUIS Category 2: Combined immunodeficiency with associated or syndromic features ^[4]^[7]^[8]

Condition	Gene / Inheritance	Key Distinguishing Features	Why It Mimics SCID	How to Differentiate
DiGeorge syndrome (22q11.2 deletion)	TBX1 region, AD (de novo microdeletion at 22q11.2)	Classical triad: Conotruncal cardiac anomalies + Hypoplastic thymus (T-cell deficiency) + Hypoplastic parathyroid (hypocalcaemia) ^[2]. Palatal abnormalities, developmental delay, characteristic facies	T-cell deficiency → susceptibility to viral/fungal infections; absent thymic shadow on CXR — looks like SCID	Cardiac anomalies (ToF, interrupted aortic arch, truncus arteriosus), neonatal hypocalcaemia, palatal cleft, characteristic facies (long face, tubular nose, small mouth). FISH or microarray for 22q11.2 deletion. T cells are ↓ but usually not absent (partial DiGeorge has some T cells; complete DiGeorge is rare and phenotypically overlaps with SCID)
Wiskott-Aldrich syndrome (WAS)	WAS gene, XLR	Tetrad: Eczema + Micro-thrombocytopenia (low AND small platelets) + Immunodeficiency + Bloody diarrhoea (secondary to thrombocytopenia) ^[4]	Recurrent infections + FTT in a boy → looks like SCID. T-cell function declines over time	Micro-thrombocytopenia is pathognomonic — platelets are both low in number AND small (low MPV), which is the opposite of ITP (where MPV is high). Low platelets + Low IgM ^[4]. Eczema is prominent.
Hyper-IgE syndrome (HIES / Job syndrome)	STAT3 (AD, loss of function) ^[4]	High IgE, high absolute eosinophil count, CXR showing massive pneumatoceles ^[4]. Bacteria (massive cold Staph abscess), Fungi ^[4]	Recurrent severe staphylococcal infections in infancy	"Cold" abscesses (lack of warmth/erythema because Th17 response is defective → poor neutrophil recruitment → pus forms without classical inflammation). Coarse facies, retained primary teeth, scoliosis. IgE is sky-high (often > 2000 IU/mL). T cells are present but Th17 is specifically deficient.
Ataxia-telangiectasia (AT)	ATM gene, AR	Progressive cerebellar ataxia + oculocutaneous telangiectasias + immunodeficiency + radiosensitivity + malignancy risk	Combined T and B cell dysfunction → recurrent sinopulmonary infections	Ataxia becomes apparent when child begins to walk (age 1–2 years). Telangiectasias appear on conjunctivae/ears by age 3–6. Elevated AFP. Later onset than SCID.
Omenn syndrome	Hypomorphic RAG1/RAG2 (or other SCID genes), AR	Erythroderma, lymphadenopathy, hepatosplenomegaly, eosinophilia, ↑↑IgE, oligoclonal T cells	Actually a "leaky" form of SCID — presents similarly but with inflammatory/autoimmune features	Distinguished from typical SCID by presence of oligoclonal T cells (not absent), eosinophilia, elevated IgE, and prominent skin disease. Genetically confirmed by finding hypomorphic mutations in SCID genes.

High Yield — DiGeorge vs SCID

Both DiGeorge and SCID can present with absent thymic shadow and T-cell lymphopenia. The key differentiator is that DiGeorge has extrahymic features: cardiac anomalies, hypocalcaemia, palatal defects, and characteristic facies. DiGeorge = defective development of pharyngeal pouch system due to heterozygous chromosomal deletion at 22q11.2 ^[2]. Most DiGeorge patients have partial T-cell deficiency (enough T cells for reasonable, though impaired, immunity), whereas SCID has near-zero T cells.

These present with sinopulmonary and GI infections ^[1] but usually later than SCID (after 4–6 months when maternal IgG wanes), and T-cell immunity is intact.

Condition	Gene / Inheritance	Key Distinguishing Features	Why It Might Be Confused with SCID	How to Differentiate
X-linked (Bruton) agammaglobulinemia (XLA)	BTK (X-linked, loss of function) ^[4]	↓B cells, pan-hypogammaglobulinaemia, absent tonsils ^[1]. Recurrent sinopulmonary and GI infections with encapsulated bacteria	Boy with recurrent infections and absent lymphoid tissue → could look like SCID	T cells are normal — XLA patients handle viruses and fungi normally (no PJP, no disseminated CMV). B cells are ↓↓↓ but T cells are intact. Treatment: IVIG / SCIG ^[4]
Common variable immunodeficiency (CVID)	Various (TNFRSF13B/TACI in 10–15%), most unknown	Most common form of severe antibody deficiency ^[1]. ↓↓↓IgG, ↓IgA/E ^[1]. Highly variable presentation	Recurrent infections + FTT → could look like SCID in a young child	Typically presents later (childhood to young adulthood). T cells are usually normal or mildly low. B cells are present in number but dysfunctional. May have autoimmune cytopenias, granulomatous disease, lymphoproliferation.
Hyper-IgM syndrome (HIGM)	CD40L (TNFSF5), XLR (Type 1)	Failure to switch IgM to IgG/A → ↑IgM, ↓IgG/A/E ^[1]	Recurrent infections in a boy; can get opportunistic infections (PJP) because CD40L mutation also impairs T-cell activation of macrophages	IgM is elevated (not low). Can present with PJP and Cryptosporidium cholangiopathy — which overlaps with SCID presentation. But B cells and T cells are present (though T-cell function is partially impaired).
Selective IgA deficiency	Various, often polygenic	Most common PID ^[1]. Usually asymptomatic ^[1]	Very unlikely to be confused with SCID	Mild or no infections. IgG and IgM are normal. Not severe.

High Yield — Timing of Presentation Distinguishes Antibody Deficiencies from SCID

Humoral (antibody) deficiencies commonly present after 4–6 months due to maternal IgG depletion ^[1]. SCID presents earlier (2–6 months) because T-cell deficiency manifests as soon as the infant encounters viruses and fungi — and maternal IgG does not protect against these. If an infant is getting PJP, disseminated CMV, or persistent candidiasis at 2–3 months, think SCID. If a 9-month-old is getting recurrent otitis media and pneumonia with encapsulated bacteria, think antibody deficiency.

Condition	Gene / Inheritance	Key Distinguishing Features	Why It Might Be Confused with SCID	How to Differentiate
Chronic granulomatous disease (CGD)	CYBB (X-linked), CYBA, NCF1, NCF2, NCF4 (AR) ^[4]	Phagocytes cannot produce reactive oxygen species (ROS) via NADPH oxidase → cannot kill catalase-positive organisms	Recurrent severe bacterial/fungal infections + granuloma formation + hepatosplenomegaly in a boy → could be mistaken for CID	Lymphocyte counts and immunoglobulins are NORMAL (CBC with differential WCC was normal but IgG was higher than normal — as in AOS Q9 ^[3]). Specific organisms: S. aureus, Aspergillus, Nocardia, Serratia, Burkholderia. BCG scar can be enlarged ^[3]. Diagnosed by dihydrorhodamine (DHR) flow cytometry or nitroblue tetrazolium (NBT) test.
Leucocyte adhesion deficiency (LAD)	ITGB2 (LAD-1), AR	Neutrophils cannot migrate to sites of infection → delayed umbilical cord separation, recurrent skin/soft tissue infections without pus formation	Recurrent bacterial infections in infancy	Leucocytosis with very high neutrophil count (neutrophils cannot leave the bloodstream). No pus at infection sites despite severe infection. Omphalitis with delayed cord separation.

AOS Exam Point — CGD vs SCID

A common AOS/exam trap is differentiating CGD from SCID. The AOS question (Q9) ^[3] described a boy with recurrent S. aureus cervical lymphadenitis, enlarged BCG scar, perianal sepsis, hepatosplenomegaly — but normal CBC and elevated IgG. Answer: Chronic granulomatous disease ^[3], NOT SCID. In SCID, the ALC would be low (< 2500/µL) ^[2]^[6], and you would NOT see elevated IgG. In CGD, the lymphocytes and immunoglobulins are intact (in fact, IgG may be elevated due to chronic immune stimulation).

Complement deficiencies can present with recurrent infections (particularly with Neisseria for terminal complement defects, or encapsulated bacteria for C3 deficiency)
They do NOT cause the severe viral/fungal/opportunistic infection pattern seen in SCID
Lymphocyte counts and immunoglobulins are normal
Distinguished by complement studies (CH50, AP50, individual complement levels)

Before diagnosing a PID, secondary causes must be considered:

Secondary Cause	Mechanism	How to Differentiate from SCID
HIV infection (vertical transmission)	HIV infects CD4⁺ T cells → progressive CD4 depletion → secondary combined immunodeficiency	Maternal HIV status should be checked in all infants with suspected immunodeficiency. HIV PCR (DNA or RNA) in infant. In SCID, T cells are absent from birth; in HIV, the infant may have normal T cells initially that decline over months. Hong Kong has low but non-zero vertical transmission rates.
Prematurity / Neonatal ICU factors	Immature immune system + breached barriers (intubation, lines) + nosocomial pathogen exposure	Clinical context is obvious. Lymphocyte counts may be low due to prematurity but should recover. Not truly T-cell absent.
Maternal immunosuppressive drugs	Transplacental passage of biologics (e.g., infliximab, rituximab) or immunosuppressants → transient neonatal immunosuppression	History of maternal drug use. Effects are transient and resolve as drug is cleared. Rituximab causes transient B-cell depletion in infant.
Protein-losing enteropathy / Severe malnutrition	Loss of immunoglobulins in stool (PLE) or inadequate protein/micronutrient intake → secondary immunodeficiency	Oedema, hypoalbuminaemia. May have low immunoglobulins but T-cell counts are preserved (or only mildly low).
Congenital infections (TORCH)	CMV, rubella, toxoplasmosis can cause transient immune dysregulation	Specific TORCH screen. Other congenital features (microcephaly, chorioretinitis, hepatosplenomegaly, petechiae).
Haematological malignancy (infant leukaemia)	Bone marrow infiltration → pancytopenia → secondary immunodeficiency	Acute lymphoblastic leukaemia can present with pancytopenia, infections, and FTT ^[3]. Blasts on peripheral blood film distinguish it. Lymphocyte count may be low (due to marrow failure) or high (if blasts are counted as "lymphocytes").
Aplastic anaemia	Bone marrow failure → pancytopenia	Pancytopenia with NO lymphadenopathy, NO hepatosplenomegaly ^[9]^[10]. Reticulocytopenia. Hypocellular marrow on biopsy. In SCID, it is specifically T cells that are absent (not all cell lines).

Feature	SCID	DiGeorge	WAS	XLA	CVID	CGD	HIV
Age at onset	< 6 mo	Birth	< 6 mo	4–6 mo	> 1 yr	< 1 yr	Variable
T cells	↓↓↓/absent	↓ (partial)	↓ (progressive)	Normal	Normal/↓	Normal	↓ (progressive)
B cells	Variable	Normal	Normal	↓↓↓/absent	Normal	Normal	Normal
Immunoglobulins	All ↓	Variable	↓IgM, ↑IgE	All ↓	↓↓IgG, ↓IgA	Normal/↑	↓ (late)
ALC	< 2500/µL ^[2]^[6]	Low-normal	Low-normal	Normal	Normal	Normal	Low (late)
Thymic shadow	Absent ^[1]^[2]	Absent	Present	Present	Present	Present	Present
Key infections	PJP, Candida, BCG, viruses ^[4]^[6]	Viral, fungal	Encapsulated bacteria	Encapsulated bacteria	Encapsulated bacteria	S. aureus, Aspergillus	Opportunistic (TB, PJP)
Unique features	None (no syndromic features)	Cardiac + hypoCa²⁺ + palatal	Micro-thrombocytopenia, eczema ^[4]	Absent tonsils ^[1]	Autoimmunity, granulomas	Normal CBC, elevated IgG ^[3]	Maternal HIV status
Inheritance	XLR or AR	AD (de novo)	XLR	XLR	Variable	XLR or AR	Acquired
Treatment	HSCT ^[4]^[5]^[6]	Thymic transplant (complete), supportive (partial)	HSCT	IVIG/SCIG ^[4]	IVIG	Septrin + IVIG ^[4]	ART

Once SCID is confirmed (near-zero T cells, absent thymic output), the next step is subtype classification by T/B/NK phenotype, confirmed by genetic testing.

Phenotype	Genetic Cause	Inheritance	Distinguishing Clue
T⁻B⁺NK⁻	IL2RG	X-linked ^[4]^[6]	Most common (~45%). Boy. No NK cells.
T⁻B⁺NK⁻	JAK3	AR	Phenocopy of X-linked SCID but can affect girls
T⁻B⁺NK⁺	IL-7Rα	AR	NK cells present (spared because IL-15/γc intact)
T⁻B⁻NK⁺	RAG1/RAG2, Artemis	AR	Both T and B cells absent (V(D)J recombination failure). NK spared (germline receptors)
T⁻B⁻NK⁻	ADA	AR	All lymphocytes absent (toxic metabolite). Costochondral abnormalities.
T⁻B⁻NK⁻	Reticular dysgenesis (AK2)	AR	Most severe: + agranulocytosis + sensorineural deafness

Omenn syndrome is a "leaky" SCID variant (usually hypomorphic RAG1/2) — distinguished by presence of oligoclonal T cells, erythroderma, eosinophilia, ↑↑IgE, and lymphadenopathy/hepatosplenomegaly (unlike classic SCID which has absent lymphoid tissue) ^[4].

6. Approach to the Infant with Suspected SCID — Clinical Reasoning

When you encounter an infant with features suggesting SCID, think through these layers:

Red flags: Opportunistic infections (PJP, Candida deep tissue), complications from a live vaccine ^[2], FTT, family history of early death
Against PID: Mild viral URTIs that self-resolve, normal growth, normal lymphocyte count

Always check: maternal HIV status, medication history (maternal biologics), nutritional status, peripheral blood film (blasts → leukaemia)
HIV vertical transmission must be excluded in all infants with suspected combined immunodeficiency

Combined deficiency (SCID): T cells absent/very low, infections with any organism (viral, fungal, bacterial, mycobacterial), onset < 6 months
Antibody deficiency: T cells normal, B cells ↓ or Ig ↓, infections predominantly bacterial (encapsulated), onset 4–6 months or later
Phagocyte defect: T and B cells normal, Ig normal, infections with catalase-positive bacteria and fungi, granuloma formation
Complement deficiency: T and B cells normal, Ig normal, infections with Neisseria or encapsulated bacteria

Lymphocyte subset analysis (flow cytometry): T/B/NK profile → narrows genetic differential
Genetic testing: Confirms specific mutation → guides treatment (e.g., gene therapy only available for certain subtypes)

Common Exam Pitfalls

CGD mistaken for SCID: Both present in infancy with severe infections and hepatosplenomegaly. But in CGD, CBC and differential WCC are normal, IgG may even be elevated ^[3]. In SCID, ALC is low and T cells are absent.
DiGeorge mistaken for SCID: Both have absent thymic shadow and low T cells. But DiGeorge has cardiac anomalies and hypocalcaemia — always check calcium and cardiac history.
Omenn syndrome mistaken for atopic dermatitis or Wiskott-Aldrich: Omenn has erythroderma and eosinophilia like WAS, but Omenn has no thrombocytopenia (WAS has micro-thrombocytopenia) and near-absent B cells (WAS has normal B cells).
HIV mistaken for SCID: Both cause progressive CD4 depletion and opportunistic infections. But HIV is acquired, progressive, and maternal HIV status is usually known. HIV PCR distinguishes them.
Maternal T-cell engraftment in SCID mistaken for normal T cells: In SCID, maternal T cells can cross the placenta and engraft in the infant → detectable T cells on flow cytometry. These are maternal (not infant-derived), often oligoclonal, and may cause GvHD-like rash. Checking T-cell chimerism (HLA typing or STR analysis) distinguishes maternal from infant T cells.

BCG is given at birth in Hong Kong → BCG vaccine will disseminate in SCID ^[4]. An infant presenting with disseminated BCGosis should have SCID at the top of the differential. This is a uniquely relevant differential in Hong Kong (and other countries with universal BCG programmes) compared to countries without routine BCG.
HIV vertical transmission is uncommon in Hong Kong due to effective antenatal screening and prevention programmes, but should still be considered
Newborn screening for SCID (TREC-based) is not yet universal in Hong Kong (pilot programme stage as of 2025) — therefore, many SCID cases may still present clinically rather than being detected presymptomatically
Delayed treatment is associated with complications such as bronchiectasis ^[5] — early recognition of the differential is critical

High Yield Summary — Differential Diagnosis of SCID

The key differential is between categories of PID: SCID (combined T+B deficiency) vs. predominantly antibody deficiency (XLA, CVID) vs. phagocyte defect (CGD) vs. syndromic CID (DiGeorge, WAS). The discriminator is T-cell count and function.
SCID: absent/very low T cells, low ALC (< 2500/µL in infants), absent thymic shadow, susceptible to ALL pathogens ^[1]^[2]^[4]^[6]
DiGeorge: Partial T-cell deficiency + cardiac anomaly + hypocalcaemia + 22q11.2 deletion
WAS: Eczema + micro-thrombocytopenia + immunodeficiency + bloody diarrhoea ^[4]
XLA: ↓B cells, pan-hypogammaglobulinaemia, normal T cells, absent tonsils, encapsulated bacteria ^[1]
CGD: Normal CBC, normal/elevated Ig, catalase-positive organisms, granulomas ^[3]
Always exclude secondary causes: HIV (check maternal status), infant leukaemia (blood film), aplastic anaemia (pancytopenia), medication effects
Omenn syndrome: "Leaky SCID" with oligoclonal T cells, erythroderma, eosinophilia, ↑IgE — a SCID variant, not a separate entity

Active Recall - Differential Diagnosis of SCID

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.410, Primary Immunodeficiency — Humoral and Combined Deficiency sections) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.638, SCID, DiGeorge, Clinical Manifestations of PID) ^[3] Senior notes: AOS - Paeds.pdf (Question 9 — CGD vs SCID, Question 10 — ALL) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (Combined immunodeficiencies, WAS, Hyper-IgE, XLA sections) ^[5] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.9, Clinical Utility of Molecular Diagnostic Testing for PID) ^[6] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.18, SCID section) ^[7] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.6–7, Tables 1–9 IUIS classification) ^[8] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.22, CID with syndromic features) ^[9] Senior notes: Ryan Ho Haemtology.pdf (p.32, Aplastic anaemia — clinical features and diagnostics) ^[10] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (p.8, Aplastic anaemia — no lymphadenopathy/hepatosplenomegaly)

1. Diagnostic Criteria for SCID

Unlike many conditions with formal consensus "diagnostic criteria" (e.g., Jones criteria for rheumatic fever), SCID is diagnosed through a combination of clinical suspicion, laboratory findings, and genetic confirmation. However, international working definitions exist, particularly from the Primary Immune Deficiency Treatment Consortium (PIDTC) and the IUIS.

1.1 PIDTC Diagnostic Criteria for SCID (2022 Updated)

The most widely used operational definition classifies infants as having Typical SCID, Leaky/Atypical SCID, or Omenn Syndrome:

All of the following must be met:

Age < 2 years at presentation (though detected earlier if NBS is available)
Absent or very low T cells: CD3⁺ T-cell count < 300 cells/µL — OR — T cells present but of confirmed maternal origin (engrafted maternal T cells)
At least ONE of the following:
- Pathogenic or likely pathogenic mutation in a known SCID gene
- Absent T-cell proliferative response to phytohaemagglutinin (PHA) — i.e., T cells that are present cannot function
- Family history of SCID in a sibling

The key concept: Typical SCID = near-zero autologous T cells + functional impairment + genetic basis.

Age < 4 years at presentation
Reduced but not absent T cells: CD3⁺ T cells 300–1500 cells/µL — with evidence that these are autologous (not maternal)
T-cell function impaired: proliferative response to PHA < 10% of lower limit of normal — OR — absent naïve T cells (CD45RA⁺CD62L⁺) with restricted T-cell receptor repertoire
Pathogenic mutation in a known SCID gene

Leaky SCID occurs when the genetic mutation is hypomorphic (partial loss-of-function) — enough residual protein activity to allow some T-cell development, but not enough for normal immunity. This is important because these patients can be missed on newborn screening (TREC may be low-normal rather than absent).

Meets criteria for Leaky/Atypical SCID, PLUS:
Generalised erythroderma (whole-body red, thickened skin)
Elevated IgE (often massively)
Eosinophilia (> 1000/µL)
Oligoclonal T cells (restricted TCR repertoire — a few T-cell clones have expanded but are autoreactive)
Hepatosplenomegaly and/or lymphadenopathy (unlike typical SCID which has absent lymphoid tissue)

High Yield — Why Omenn Syndrome Looks Different from Typical SCID

In typical SCID, there are essentially no T cells → no lymphoid tissue expansion, no inflammation. In Omenn syndrome, hypomorphic mutations (usually in RAG1/RAG2) allow a few T-cell clones to develop. These oligoclonal T cells escape normal selection → they are autoreactive → they cause a systemic inflammatory response (erythroderma, eosinophilia, high IgE) that looks paradoxically like an immune "over-activation" despite the patient being profoundly immunodeficient.

In clinical practice, especially in Hong Kong where universal SCID NBS is not yet routine, you rely on clinical red flags to trigger the workup:

Features suggestive of primary immunodeficiency (PID): ^[2]

Patient factors:

Family history of immunodeficiency or unexplained early death (before 30)
Failure to gain weight or grow normally (failure to thrive)
Persistent lymphopenia: < 2500 cells/µL in younger children; < 1500 cells/µL in patients over 5 years ^[2]

Infection-related factors:

≥ 6 new infections within 1 year
≥ 2 serious sinus infections or pneumonias within 1 year
≥ 2 episodes of sepsis or meningitis in a lifetime
Recurrent or resistant candidiasis
Infection with an opportunistic organism
Complications from a live vaccine ^[2]

Treatment-related factors:

≥ 2 months of antibiotics with little effect
Need for IV antibiotics or hospitalisation to clear infections ^[2]

The algorithm has three phases: (1) Clinical suspicion / Newborn screening, (2) Confirmatory immunological workup, (3) Genetic diagnosis.

2.1 Mermaid Diagnostic Algorithm

3. Investigation Modalities — Detailed

3.1 Phase 1: Screening / Initial Investigations

Low TREC (T-cell Receptor Excision Circle) on newborn screening ^[4]^[6]

Feature	Detail
What is TREC?	A small circular DNA by-product generated during T-cell receptor (TCR) α-chain gene rearrangement in the thymus. Every time a new T cell is produced, one TREC is generated. It is NOT replicated during cell division, so it is a marker of recent thymic output (new T-cell production).
Sample	Dried blood spot (DBS) — the same Guthrie card used for other NBS tests (e.g., PKU, CH)
Method	Real-time quantitative PCR (qPCR) to amplify and quantify TREC DNA
Normal	Varies by laboratory; typically > 20–25 copies/µL on DBS
In SCID	Very low or undetectable (< 10 copies/µL or absent) — because the thymus is not producing T cells
Hong Kong context	TREC-based NBS is in pilot phase (not yet universal as of 2025). Most cases still present clinically.
False positives	Prematurity (immature thymus → low TREC that improves), DiGeorge syndrome (22q11.2 deletion), congenital heart disease with thymic stress, secondary T-cell lymphopenia (e.g., chylous effusions, third-spacing). These require follow-up but are not SCID.
False negatives	Leaky SCID or Omenn syndrome (some T-cell production → borderline TREC), maternal T-cell engraftment (maternal T cells may contribute TRECs if recently produced)

High Yield — Why TREC is the Ideal Screening Biomarker

TREC is ideal for NBS because: (1) it is measured from the same DBS already collected for other NBS tests — no additional blood draw needed; (2) it detects all forms of SCID regardless of the genetic subtype (because all share absent/low thymic output); (3) it is quantitative via PCR — cheap, fast, and scalable. It does NOT directly measure T-cell count — it measures thymic output, which is a more upstream indicator.

Low ALC (< 2500/mL) — ALC low < 3000 (< 1y) or < 1000 (> 1y) → severe combined immunodeficiencies ^[4]^[6]^[11]

Feature	Detail
Why ALC?	The CBC is the single most accessible initial test. In neonates and young infants, physiological lymphocytosis is normal (ALC typically 4,000–10,000/µL). An ALC that appears "normal" for adults (e.g., 1,800/µL) is critically low in an infant.
Age-specific thresholds	ALC < 3000/µL in infants < 1 year is suspicious; ALC < 1000/µL in children > 1 year ^[11]. < 2500/µL in younger children ^[2] is the threshold used in many PID screening guidelines
In SCID	ALC is typically < 1000/µL and often < 500/µL — dramatically below neonatal norms
Caveat	ALC alone does NOT distinguish which lymphocyte subset is affected. Need flow cytometry for T/B/NK subsets. Also, maternal T-cell engraftment can raise the ALC slightly in SCID (misleading if not checking subsets).

Complete blood count findings in SCID: ^[11]

ALC low < 3000 (< 1y) or < 1000 (> 1y) → severe combined immunodeficiencies

AEC high → primary atopic disorders

ANC < 1 → severe congenital neutropenia

ANC high → leukocyte adhesion deficiency

PLT low → Wiskott-Aldrich syndrome

GC Lecture Slide — Key CBC Findings in PID

The GC 144 lecture slide ^[11] provides a one-stop summary of CBC patterns in different PIDs. This is very high yield for in-house exams — each cell-line abnormality points to a specific PID category. Memorise the associations: low ALC → SCID; low platelets → WAS; high ANC → LAD; low ANC → severe congenital neutropenia.

Thymus shadow is absent on CXR ^[1]^[2]

Feature	Detail
Normal neonatal thymus	Large, prominent — produces a "sail sign" on AP CXR (triangular opacity projecting from the mediastinum). The thymus is largest relative to body size in the neonatal period.
In SCID	Absent thymic shadow — the thymus is hypoplastic/involuted because there are no T-cell precursors to populate it
Why this matters	An absent thymic shadow in a well neonate on routine CXR should trigger consideration of SCID. Conversely, a visible thymus in a sick infant essentially rules out typical SCID (the thymus would be expected).
Caveat	The thymic shadow can be obscured by overlying structures or may appear small on a poorly positioned AP film. In stressed or sick neonates, the thymus can transiently involute (e.g., after surgery, infection, steroids) — this is NOT the same as congenital absence. Clinical context is key.

3.2 Phase 2: Confirmatory Immunological Workup

Once screening tests suggest SCID (low TREC, low ALC, absent thymic shadow), the following investigations confirm the diagnosis and classify the subtype.

Lymphocyte subset — low T cells (with or without low B and/or NK) → combined immunodeficiencies ^[11]

This is the cornerstone investigation for diagnosing and classifying SCID.

Marker	What It Measures	Expected in SCID
CD3⁺	Total T cells	Low / zero T-cell count ^[4]^[6] — typically < 300/µL in typical SCID
CD4⁺	Helper T cells	Very low / absent
CD8⁺	Cytotoxic T cells	Very low / absent
CD19⁺ (or CD20⁺)	B cells	Variable — depends on subtype (present in T⁻B⁺ SCID; absent in T⁻B⁻ SCID)
CD16⁺/CD56⁺	NK cells	Variable — absent in T⁻B⁺NK⁻ SCID (IL2RG, JAK3); present in T⁻B⁺NK⁺ or T⁻B⁻NK⁺ SCID
CD45RA⁺ on CD4⁺ T cells	Naïve T cells (recent thymic emigrants)	Low naïve T cells ^[4] — this is critical for distinguishing true SCID from maternal T-cell engraftment

Why Naïve T Cells Matter — Maternal T-Cell Engraftment Trap

In some SCID infants, maternal T cells cross the placenta and engraft in the infant. On flow cytometry, these appear as CD3⁺ T cells, potentially giving a falsely reassuring count. However, these maternal T cells are memory T cells (CD45RO⁺), NOT naïve T cells (CD45RA⁺). Therefore: Low naïve T cells ^[4] with detectable total T cells → suspect maternal engraftment → confirm with HLA typing or STR chimerism analysis (the T cells will have the mother's HLA type, not the infant's). In some forms of CID, T cells may be normal in count but not in function, or they may be maternally grafted memory T cells in Omenn syndrome, a form of SCID ^[11].

The T/B/NK phenotype immediately narrows the genetic differential:

Phenotype	Genetic Suspects	Next Test
T⁻B⁺NK⁻	IL2RG (XL), JAK3 (AR)	Gene sequencing of IL2RG first (if male); JAK3 if female or IL2RG negative
T⁻B⁺NK⁺	IL-7Rα, CD3δ/ε/ζ, CD45	Gene panel
T⁻B⁻NK⁺	RAG1, RAG2, Artemis, LIG4, NHEJ1	Gene panel; radiation sensitivity testing for Artemis
T⁻B⁻NK⁻	ADA, PNP, AK2 (reticular dysgenesis)	Measure ADA enzyme activity (red cell ADA); metabolites (dAXP); gene panel

IgGAME findings: ^[11]

Low IgGAM → agammaglobulinaemia (e.g., XLA)
High IgE → Hyper-IgE syndrome, primary atopic disorder
High IgM → Hyper-IgM syndrome
High IgGAM → chronic granulomatous disease

Immunoglobulin	Expected in SCID	Interpretation
IgG	May be normal or low in first 3 months (maternal transplacental IgG is still present). After 3–6 months → low as maternal IgG wanes and infant cannot produce its own	Must interpret IgG in context of age and maternal IgG contribution. A "normal" IgG in a 2-month-old may be entirely maternal.
IgA	Low or absent — IgA does not cross the placenta, so any IgA present must be infant-produced. In SCID, no T-cell help → no class switching → absent IgA from birth	Low IgA in a neonate is more informative than low IgG for SCID diagnosis
IgM	Low or absent — like IgA, IgM does not cross the placenta. Infant IgM production begins in utero but requires T-cell signals for optimal response	IgM is the first immunoglobulin produced by the infant. Very low IgM in a neonate with low T cells strongly suggests SCID
IgE	Low or absent in typical SCID. Elevated in Omenn syndrome (oligoclonal autoreactive T cells drive IgE switching)	Elevated IgE in an infant with erythroderma + eosinophilia → think Omenn syndrome

High Yield — Immunoglobulin Pattern Differentiates PID Types

Pattern	PID	Why
Low IgGAM	XLA, SCID	No B cells (XLA) or no T-cell help for B cells (SCID)
High IgM, low IgG/A/E	Hyper-IgM syndrome	Cannot class-switch from IgM
High IgGAM	CGD ^[11]	Intact adaptive immunity but chronic immune stimulation from persistent infections
High IgE	Hyper-IgE, Omenn, DOCK8	Th2 skewing or oligoclonal T-cell activation

Test	Method	Expected in SCID	Clinical Utility
Mitogen stimulation (PHA, ConA, PWM)	Patient's lymphocytes are cultured with plant lectins (mitogens) that non-specifically activate T cells. Proliferation is measured (by thymidine incorporation or CFSE dilution).	Absent or severely reduced proliferation — even if some T cells are present (e.g., maternal engraftment or Omenn), they often fail to proliferate normally	Confirms functional T-cell deficiency. A patient can have detectable T cells on flow cytometry but zero function → still SCID. This is part of the PIDTC criteria.
Specific antigen stimulation (Candida, tetanus toxoid)	Tests memory T-cell response to previously encountered antigens	Absent response (infant has not been vaccinated or cannot mount memory response)	Less useful in neonates (no prior antigen exposure). More relevant in older infants/children.

Test	Method	Purpose
HLA typing	Flow cytometry or molecular HLA typing of sorted T cells vs. infant's own cells (buccal swab or fibroblasts)	If T cells carry maternal HLA alleles not present in the infant's germline → confirms maternal engraftment
Short tandem repeat (STR) analysis	Microsatellite DNA analysis on sorted T cells	Gold standard for chimerism detection; can quantify % maternal vs. autologous T cells

This is critical because maternal T cells in SCID can cause graft-versus-host disease (GvHD) — presenting as erythroderma, hepatitis, or colitis — which can be confused with Omenn syndrome or infection.

3.3 Phase 3: Genetic Diagnosis

Molecular basis of many (PIDs) has been worked out — mostly single gene disorders ^[5]

Most common first-line genetic test now
A panel of > 20 known SCID genes is sequenced simultaneously using next-generation sequencing (NGS)
Typical turnaround: 2–4 weeks
Identifies pathogenic variants in IL2RG, JAK3, IL7R, RAG1, RAG2, DCLRE1C, ADA, PNP, AK2, CD3D, CD3E, CD3Z, PTRCRA, CORO1A, LIG4, NHEJ1, etc.

Used when targeted panel is negative but clinical picture is convincing
Can identify novel SCID genes or atypical mutations
Increasingly used as a first-line test in specialised centres

Test	Method	Expected in ADA-SCID
Red cell ADA enzyme activity	Spectrophotometric assay on haemolysate	Absent or profoundly reduced (< 1% of normal)
Erythrocyte dATP (deoxyadenosine triphosphate)	HPLC or tandem mass spectrometry	Elevated (toxic metabolite that accumulates in ADA deficiency)
Total purine metabolites in urine	Urinary deoxyadenosine	Elevated

ADA-SCID is the one subtype where an enzyme assay can provide rapid, specific diagnosis before genetic results return. This matters because enzyme replacement therapy (PEG-ADA) can be started immediately as a bridge to HSCT.

Low KREC on newborn screening — KREC is just the B-cell counterpart for TREC (Kappa-deleting recombination excision circles) ^[4]

KRECs are DNA circles produced during kappa light chain gene rearrangement in B cells
Low KREC indicates impaired B-cell development (e.g., XLA, T⁻B⁻ SCID)
Combined TREC + KREC screening on NBS can detect both T-cell and B-cell defects simultaneously
Currently in research/pilot phase in most centres including Hong Kong

3.4 Phase 4: Workup for Complications and Pre-HSCT Assessment

Once SCID is confirmed, additional investigations are needed to assess the infant's current infection burden and fitness for HSCT:

Investigation	Purpose	Expected Findings
Blood cultures (bacterial, fungal)	Detect bacteraemia/fungaemia	May grow unusual organisms
CMV PCR (blood)	Detect CMV viraemia	Positive in many SCID infants (acquired perinatally); critical to know pre-HSCT as CMV status affects conditioning and prognosis
EBV PCR (blood)	Detect EBV viraemia	May be positive; risk of lymphoproliferative disease
Adenovirus PCR	Common cause of severe disease in SCID	May be positive
RSV / Parainfluenza NPA	Respiratory viral panel	May be positive; respiratory viral infection pre-HSCT worsens outcomes
PJP workup (induced sputum or BAL for PJP PCR/stain)	Diagnose Pneumocystis pneumonia	PJP is a classic presentation of SCID
Stool for viruses, ova, cysts, parasites	Assess enteric pathogen burden	Rotavirus, norovirus, Cryptosporidium
BCG site assessment	Assess for BCGitis/BCGosis	Ulceration, regional lymphadenopathy, disseminated disease
AFB cultures / Mycobacterial PCR	Confirm mycobacterial infection from BCG	Positive for M. bovis BCG

HIV DNA PCR in all infants with suspected SCID — to exclude vertical HIV infection which can mimic SCID
In Hong Kong, maternal HIV is screened antenatally, but always verify

Investigation	Purpose
HLA typing (high-resolution) of patient, parents, and siblings	Identify optimal HSCT donor (HLA-matched sibling > haploidentical parent > MUD)
Baseline organ function (LFT, RFT, echocardiogram)	Assess fitness for conditioning chemotherapy
CMV/EBV/Adenovirus status of patient and donor	Guide post-transplant monitoring
Maternal T-cell chimerism	Quantify maternal engraftment (may cause GvHD post-HSCT)

Phase	Investigation	Key Finding in SCID	Rationale
Screening	TREC (NBS)	Low TREC ^[4]^[6]	Absent thymic output
Screening	CBC + differential	ALC < 2500/µL (infant) ^[2]; < 3000 (< 1y) ^[11]	Lymphopenia from absent T cells
Screening	CXR	Absent thymic shadow ^[1]^[2]	No T-cell precursors → thymic hypoplasia
Confirmatory	Lymphocyte subsets (flow cytometry)	Low/zero T-cell count ^[4]^[6]; classify T/B/NK	Identifies and classifies SCID
Confirmatory	Naïve T cells (CD45RA⁺)	Low naïve T cells ^[4]	Distinguishes autologous from maternal T cells
Confirmatory	Immunoglobulins (IgG/A/M/E)	Low IgA, IgM; IgG may be falsely normal (maternal)	Confirms humoral deficiency
Confirmatory	T-cell proliferation (PHA)	Absent/severely reduced	Confirms functional T-cell deficiency
Confirmatory	Maternal chimerism (HLA/STR)	Maternal T cells detected	Identifies maternal engraftment
Genetic	Targeted gene panel / WES	Pathogenic variant identified	Definitive diagnosis; guides treatment
Genetic	ADA enzyme activity	Absent in ADA-SCID	Rapid specific diagnosis; start ERT
Complications	Infection screen (CMV, EBV, PJP, BCG)	Active infections common	Pre-HSCT assessment
Pre-HSCT	HLA typing (family)	Match donor identified	Plan HSCT

4. Interpreting Key Results — Worked Examples

3-month-old boy, FTT, persistent oral thrush, CXR: absent thymic shadow.

CBC: WCC 4.0, ALC 600/µL (critically low for age)

Flow cytometry: CD3 = 50/µL (↓↓↓), CD19 = 800/µL (normal), CD16/56 = 20/µL (↓↓)

Immunoglobulins: IgG 2.1 g/L (may be maternal), IgA < 0.05 g/L (↓), IgM 0.08 g/L (↓)

PHA proliferation: No response

Interpretation: T⁻B⁺NK⁻ → most likely IL2RG (X-linked) ^[4]^[6] → confirm with gene sequencing

2-month-old girl, FTT, bilateral PJP pneumonia, CXR: absent thymic shadow + costochondral junction abnormalities.

CBC: ALC 200/µL

Flow cytometry: CD3 = 30/µL (↓↓↓), CD19 = 40/µL (↓↓), CD16/56 = 10/µL (↓↓)

Interpretation: T⁻B⁻NK⁻ → suspect ADA deficiency

ADA enzyme activity on RBC: Undetectable

dATP in red cells: Markedly elevated

Action: Start PEG-ADA enzyme replacement immediately as a bridge to HSCT, while genetic confirmation proceeds

4-month-old boy, generalised erythroderma, hepatosplenomegaly, lymphadenopathy.

CBC: ALC 3000/µL (normal-appearing, but...), eosinophils 4500/µL (↑↑)

Flow cytometry: CD3 = 2500/µL — but almost all are CD45RO⁺ (memory), CD45RA⁺ (naïve) = 50/µL

IgE: 12,000 IU/mL (↑↑↑), IgG/A/M: very low

TCR spectratyping: Oligoclonal (few dominant peaks)

Interpretation: Omenn syndrome — "leaky" SCID with oligoclonal autoreactive T cells. In Omenn syndrome, T cells may be normal in count but not in function, or they may be maternally grafted memory T cells ^[11]

Genetic testing: Hypomorphic RAG1 mutation confirmed

Pitfall — Do NOT Be Reassured by a 'Normal' ALC in Omenn Syndrome

The ALC may be normal or even elevated in Omenn syndrome because the oligoclonal T cells proliferate extensively. The clue is that naïve T cells are absent and the T-cell repertoire is restricted. Always request naïve T-cell markers (CD45RA) when SCID is in the differential, even if ALC looks okay.

Pitfall	Explanation	Solution
Using adult ALC reference ranges	Normal infant ALC is 4,000–10,000/µL. An ALC of 2,000 looks "normal" by adult standards but is critically low in an infant.	Always use age-appropriate reference ranges
IgG interpretation in neonates	Neonatal IgG is almost entirely maternal transplacental IgG. A "normal" IgG in a 2-month-old tells you nothing about the infant's own humoral immunity.	Check IgA and IgM (which do NOT cross the placenta) for the infant's own Ig production. Recheck IgG at 4–6 months when maternal IgG has waned.
Confusing thymic absence with thymic involution	The thymus can involute transiently in sick, stressed, or steroid-treated infants (physiological thymic involution). This is NOT the same as congenital thymic absence in SCID.	Interpret CXR thymic shadow in clinical context. If clinical picture suggests SCID, do not dismiss an absent thymus as "stress involution."
Missing SCID because of maternal T-cell engraftment	Maternal T cells can produce a falsely reassuring T-cell count	Check naïve T cells (CD45RA) and maternal chimerism
TREC false positives on NBS	Premature infants often have low TREC due to immature thymus	Recheck TREC after discharge or at corrected term age. Reflex to lymphocyte subsets if persistently low.

Molecular basis of many PIDs has been worked out — mostly single gene disorders. Several are X-linked, accounting for male preponderance of some PID. Many are treatable — HSC transplantation for Severe combined immunodeficiency. Delayed treatment associated with complications such as bronchiectasis. ^[5]

Genetic testing is not just academic — it has direct clinical implications:

Genetic Diagnosis	Clinical Implication
IL2RG confirmed	X-linked recessive → genetic counselling (50% of mother's sons affected). Gene therapy is an option in addition to HSCT.
ADA confirmed	PEG-ADA enzyme replacement can be started immediately. Gene therapy (Strimvelis / lentiviral-based) is also available.
RAG1/2 confirmed	Standard HSCT. Watch for Omenn phenotype. No gene therapy currently available.
Artemis confirmed	Radiation-sensitive → reduced-intensity conditioning (RIC) mandatory for HSCT (standard myeloablative conditioning causes unacceptable toxicity due to DNA repair defect)
AK2 (reticular dysgenesis)	HSCT is the only option; myeloid reconstitution is also needed (not just lymphoid)

High Yield — Why Genetic Diagnosis Changes Management

Delayed treatment is associated with complications such as bronchiectasis ^[5]. Early genetic diagnosis allows early HSCT (best outcomes if done < 3.5 months of age, before onset of infections). Knowing the specific gene also determines: (1) whether gene therapy is available, (2) whether enzyme replacement can bridge, (3) whether reduced-intensity conditioning is required (Artemis), and (4) genetic counselling for future pregnancies.

Step	Action	Key Test	Key Finding
1	Suspect SCID	Clinical red flags OR low TREC on NBS	FTT + opportunistic infections + live vaccine complication, OR TREC < cut-off
2	Confirm lymphopenia	CBC with differential	ALC < 2500/µL (< 1y) or < 1000 (> 1y) ^[2]^[11]
3	Assess thymus	CXR (AP)	Absent thymic shadow ^[1]^[2]
4	Classify lymphocyte subsets	Flow cytometry (CD3/4/8/19/16/56)	Low/zero T-cell count ^[4]^[6]; determine T/B/NK phenotype
5	Confirm T-cell dysfunction	PHA proliferation + naïve T cells (CD45RA)	Absent proliferation; low naïve T cells ^[4]
6	Quantify humoral immunity	Serum IgG/A/M/E	Low IgA/M (infant's own); IgG may be maternal
7	Exclude secondary causes	HIV PCR, blood film	Rule out HIV, leukaemia
8	Check for maternal engraftment	HLA/STR chimerism	Maternal T cells identified (if present)
9	Genetic confirmation	Targeted gene panel / WES	Pathogenic variant identified → definitive diagnosis
10	Pre-treatment workup	Infection screen, HLA typing (family), organ function	Guide HSCT planning

High Yield Summary — Diagnosis of SCID

Screening: Low TREC on NBS ^[4]^[6] (or clinical suspicion in non-NBS settings) + ALC < 2500/µL in infants ^[2] + absent thymic shadow on CXR ^[1]^[2]
Confirmation: Flow cytometry showing low/zero T-cell count ^[4]^[6], low naïve T cells ^[4]; absent T-cell proliferation to PHA; low immunoglobulins (especially IgA and IgM)
Classification: T/B/NK phenotype narrows the genetic differential — T⁻B⁺NK⁻ → IL2RG (X-linked, most common) ^[4]^[6]
Genetic diagnosis: Targeted panel or WES confirms specific mutation → guides treatment selection
Many PIDs are treatable — HSC transplantation for SCID. Delayed treatment associated with complications such as bronchiectasis ^[5]
Paediatric pitfalls: Always use age-appropriate ALC ranges; neonatal IgG is maternal; naïve T cells distinguish SCID from maternal engraftment and Omenn syndrome; in Omenn/SCID, T cells may be maternally grafted memory T cells ^[11]

Active Recall - SCID Diagnosis and Investigations

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.410, SCID — Ix: absent thymus shadow on CXR, low ALC) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.638, SCID diagnostic features, PID clinical manifestation warning signs) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (SCID tests — low TREC, low ALC, low/zero T-cell count, low naïve T cells; KREC explanation; immunoglobulin patterns across PIDs) ^[5] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.9, Molecular diagnostic testing for PID — single gene disorders, X-linked, treatable, delayed treatment complications) ^[6] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.18, SCID — IL2RG, tests: low TREC, low ALC, low/zero T cells, low naïve T cells, treatment HSCT) ^[11] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.50, CBC findings in PID — ALC thresholds, IgGAME patterns, lymphocyte subset interpretation, Omenn syndrome caveat about maternally grafted memory T cells)

SCID is a paediatric immunological emergency. The management rests on three pillars, running in parallel:

Immediate supportive care — keep the infant alive by preventing and treating infections
Definitive curative treatment — restore a functional immune system (HSCT, gene therapy, or enzyme replacement)
Family-centred care and genetic counselling — support the family, plan for future pregnancies

SCID is fatal without treatment ^[1]. Treatment of SCID: HSCT ^[4]^[6]. Many (PIDs) are treatable — HSC transplantation for Severe combined immunodeficiency ^[5]. Delayed treatment associated with complications such as bronchiectasis ^[5].

The single most important determinant of outcome is speed: survival after HSCT is > 90% when performed before 3.5 months of age and before the onset of active infections, but drops to 50–70% when transplanted after infections are already established. This is why newborn screening matters — it gives us a head start before the clock runs out.

3. Supportive Care — "Keep the Infant Alive Until Definitive Treatment"

Supportive care begins the moment SCID is suspected — do not wait for genetic confirmation. The goal is to minimise pathogen exposure and provide passive immune support until the infant's own immune system is restored by transplant.

Measure	Rationale	Details
Strict reverse / protective isolation	SCID infants have no adaptive immune defence — any pathogen (even normally harmless) can cause fatal disease	Single room, positive-pressure airflow if available, strict hand hygiene, gowns/gloves/masks for all contacts
Avoid daycare, crowded settings, sick contacts	Respiratory viruses (RSV, parainfluenza, adenovirus) are lethal in SCID	Keep infant at home or in hospital isolation until transplant engraftment
Limit visitors	Even asymptomatic carriers (e.g., CMV-shedding toddler siblings) can transmit dangerous infections	Screen household members; ideally siblings should be up-to-date on inactivated vaccines

SCID infants need prophylaxis against the organisms they are most vulnerable to — fungi (PJP, Candida), viruses (CMV, HSV), and bacteria.

Agent	Target	Dose (Paediatric)	Rationale
Co-trimoxazole (Septrin)	PJP (Pneumocystis jirovecii) ^[4]	Trimethoprim 5 mg/kg/day PO divided BD, 3 days per week (e.g., Mon/Wed/Fri)	PJP is a classic opportunistic infection in SCID; prophylaxis is standard of care. Check G6PD status first — sulphonamide component can cause haemolysis in G6PD-deficient infants.
Fluconazole (or Nystatin PO for mild)	Candida species	Fluconazole 3–6 mg/kg/day PO once daily	Mucocutaneous and invasive candidiasis is very common in SCID (absent Th17 defence).
Aciclovir	HSV, VZV	20 mg/kg/dose PO TDS (prophylactic dose)	If HSV/VZV risk is high (e.g., known exposure, maternal history). Some centres give routinely.
(Val)ganciclovir	CMV	If CMV PCR positive: valganciclovir 16 mg/kg/dose PO BD (treatment dose)	CMV viraemia is common and must be treated pre-HSCT; reduces transplant-related mortality. Myelosuppressive — monitor CBC.

Why Co-trimoxazole for PJP Prophylaxis?

Pneumocystis jirovecii ("jirovecii" = named after the Czech parasitologist Otto Jírovec) is an atypical fungus that specifically requires CD4⁺ T-cell–mediated immunity for clearance. In SCID, CD4⁺ T cells are absent → PJP pneumonia is one of the most common presenting infections. Co-trimoxazole (trimethoprim + sulfamethoxazole) inhibits sequential steps in folate metabolism of the organism. It is cheap, effective, and the gold standard for PJP prophylaxis across all immunocompromised populations (SCID, HIV with CD4 < 200, post-transplant).

Feature	Detail
What	Intravenous immunoglobulin (IVIG) — pooled IgG from > 1000 healthy donors
Dose	0.4–0.6 g/kg every 3–4 weeks (paediatric dosing)
Route	IV infusion (hospital or home-based). Subcutaneous Ig (SCIG) can be used in older/stable patients.
Target trough IgG	Maintain trough IgG > 5 g/L (some centres target > 7 g/L)
Why	SCID infants have no functional humoral immunity — they cannot produce their own IgG (even if B cells are present, they lack T-cell help for class switching). IVIG provides passive antibody protection against encapsulated bacteria and common viruses.
Duration	Continue until immune reconstitution is confirmed post-HSCT (usually 6–12 months or longer). Some patients need lifelong IVIG if B-cell reconstitution is incomplete after HSCT.
Side effects	Headache, fever, chills (rate-related), rarely anaphylaxis (especially in IgA-deficient patients with anti-IgA antibodies — though this is more relevant to CVID/selective IgA deficiency than SCID).

Irradiated cellular blood components must be used to prevent transfusion-associated graft-versus-host disease (TA-GvHD) ^[12]^[13].

Measure	Rationale
Irradiated blood products	In SCID, the infant cannot reject donor lymphocytes in transfused blood products. These donor T cells can engraft and mount an immune response against the infant's tissues → TA-GvHD — a usually fatal complication. Gamma-irradiation (25 Gy minimum) destroys donor lymphocytes while preserving red cell and platelet function. The use of irradiated cellular blood components prevents TA-GvHD in patients with congenital immunodeficiency ^[12].
CMV-negative blood products	SCID infants cannot control CMV. Transfusion of CMV-positive blood can cause fatal disseminated CMV disease. Use CMV-seronegative donors or leucodepleted products (leucodepletion removes > 99.9% of WBCs including CMV-carrying monocytes).
Leucodepleted products	Removes donor WBCs → reduces risk of CMV transmission, febrile non-haemolytic transfusion reactions, and HLA alloimmunisation (important pre-HSCT)

High Yield — Emergency: No Irradiated Blood Available

If the patient is immunocompromised and requires irradiated blood products but you don't have it on hand in an emergency: ask the blood bank for the oldest bag of blood. Once blood is stored for more than 14 days, the lymphocytes should have died — the oldest bag of blood should in theory have the lowest risk of TA-GvHD ^[13]. This is a practical pearl for exam and clinical scenarios.

Rule	Rationale
NO live vaccines to the infant	Live vaccines (BCG, rotavirus, OPV, MMR, varicella, yellow fever) contain attenuated but viable organisms. In SCID, the infant cannot mount an immune response to control them → disseminated vaccine-strain disease. BCG vaccine will disseminate ^[4]; complications from a live vaccine are a hallmark of PID ^[2].
Avoid OPV in household contacts	Oral polio vaccine (OPV) virus can be shed in stool and transmitted to the SCID infant → paralytic polio. Use IPV (inactivated) for household members. (Note: Hong Kong uses IPV already, but be aware of OPV use in travellers from endemic regions.)
Inactivated vaccines are safe but useless	Can be given safely but the SCID infant will not mount an immune response → they serve no purpose until after immune reconstitution post-HSCT

Intervention	Indication
Breastfeeding	Generally encouraged — maternal IgA provides mucosal protection. However, if mother is CMV IgG-positive, discuss risk of CMV transmission via breast milk (CMV can reactivate in breast milk). Many centres recommend pasteurised breast milk or formula in this setting.
NG tube or parenteral nutrition	If chronic diarrhoea causes severe malabsorption / FTT and oral feeds are insufficient. Many SCID infants require supplemental enteral nutrition pre-HSCT.
Micronutrient supplementation	Standard paediatric vitamin D, iron as needed.

Active infections must be treated aggressively with targeted antimicrobials:

Infection	Treatment	Notes
PJP	IV co-trimoxazole (TMP 15–20 mg/kg/day in 3–4 divided doses) × 21 days; ± adjunctive corticosteroids if severe hypoxia	Check G6PD. IV pentamidine if intolerant.
CMV viraemia/disease	IV ganciclovir (5 mg/kg/dose BD) → step down to PO valganciclovir when stable	Monitor for neutropenia (myelosuppressive)
Disseminated BCGosis	Multi-drug anti-TB regimen: isoniazid + rifampicin + ethambutol ± fluoroquinolone (depending on sensitivity). Prolonged course (6–12+ months)	BCG is M. bovis — intrinsically resistant to pyrazinamide. Must treat throughout pre-HSCT, peri-HSCT, and post-HSCT period.
Invasive candidiasis	IV amphotericin B (liposomal, 3–5 mg/kg/day) or IV micafungin	Fluconazole may be adequate for mucocutaneous disease
Adenovirus	IV cidofovir ± oral brincidofovir	Nephrotoxic — monitor renal function
RSV / Parainfluenza	Supportive ± ribavirin (nebulised or IV in severe cases)	Respiratory viral infections pre-HSCT significantly worsen transplant outcomes — delay HSCT if possible until cleared

4. Definitive Curative Treatment

4.1 Haematopoietic Stem Cell Transplantation (HSCT)

Treatment of SCID: HSCT ^[4]^[6]. HSC transplantation for Severe combined immunodeficiency ^[5].

HSCT is the gold standard definitive treatment for the vast majority of SCID subtypes. It works by replacing the infant's defective haematopoietic system with donor-derived stem cells that can produce functional T cells, B cells, and (depending on conditioning) myeloid cells.

The concept is straightforward:

Clear space in the infant's bone marrow (conditioning — optional/variable in SCID)
Infuse donor HSCs → they home to the bone marrow, engraft, and reconstitute a new immune system
Donor T-cell progenitors migrate to the infant's thymus → undergo normal T-cell development → produce a new, functional T-cell repertoire

Donor Type	Priority	Survival	Notes
HLA-matched sibling donor (MSD)	First choice	> 90–95% if transplanted early	Ideal because HLA-identical → lowest risk of GvHD; often no conditioning needed (especially for T⁻B⁺ SCID). But only ~20% of patients have an MSD.
Matched unrelated donor (MUD)	Second choice	80–90%	From international registries (e.g., BMDW, NMDP). Requires higher-resolution HLA matching (10/10 at HLA-A, -B, -C, -DRB1, -DQB1). Takes 2–4 months to find.
Haploidentical parent donor	Third choice (but readily available)	70–85% (improving with modern T-cell depletion)	Parent shares exactly 50% of HLA with child (by definition). Requires rigorous T-cell depletion of the graft (e.g., CD34⁺ selection, TCRαβ/CD19 depletion) to prevent severe GvHD. The advantage: always available immediately (every child has at least one parent).
Unrelated cord blood (UCB)	Alternative	70–80%	From cord blood banks. Allows greater HLA mismatch tolerance (4–6/6 match acceptable). Smaller cell dose → slower engraftment.

Clinical Decision — Timing vs. Donor Match

The best outcome in SCID HSCT is achieved when transplant occurs before 3.5 months of age and before active infections develop. If an MSD is available, proceed immediately. If not, the question is: wait for a well-matched MUD (2–4 months) vs. proceed quickly with a haploidentical parent. Modern TCRαβ/CD19-depleted haploidentical transplants have significantly improved outcomes, making haploidentical HSCT a viable strategy when speed is critical and no MSD is available.

"Conditioning" refers to chemotherapy ± immunosuppression given before HSCT to:

Create space in the bone marrow for donor HSCs to engraft
Prevent rejection of the donor graft by the recipient's residual immune cells (relevant even in SCID — some infants have maternal engrafted T cells or residual NK cells)

Approach	When Used	Examples	Rationale
No conditioning / minimal conditioning	MSD transplant in T⁻B⁺ SCID (e.g., X-linked SCID)	None, or low-dose busulfan only	In T⁻B⁺ SCID, the infant has virtually no immune system to reject the graft. A MSD graft can engraft without conditioning. T-cell reconstitution is excellent. However, B-cell reconstitution may be incomplete (donor T cells engraft but donor B-cell stem cells may not home adequately without myeloablation → patient may need lifelong IVIG).
Reduced-intensity conditioning (RIC)	MUD, haploidentical, or when full B-cell reconstitution is desired	Low-dose busulfan ± fludarabine ± serotherapy (ATG or alemtuzumab)	Provides some marrow space for donor stem cell engraftment while minimising toxicity. Better chance of full B-cell (and myeloid) chimerism → patient may eventually stop IVIG.
Myeloablative conditioning (MAC)	T⁻B⁻ SCID (RAG, Artemis, ADA) where both T and B cell reconstitution is needed; MUD or cord blood transplants	Busulfan + cyclophosphamide, or busulfan + fludarabine at full doses	Fully ablates the infant's marrow → allows complete donor chimerism (T + B + myeloid). Higher toxicity but best chance of lifelong immune independence.

High Yield — Artemis SCID and Radiation Sensitivity

Infants with Artemis (DCLRE1C) deficiency have a defect in non-homologous end joining (NHEJ) — the DNA repair pathway used after double-strand breaks. Alkylating agents (busulfan, cyclophosphamide) and radiation cause double-strand DNA breaks. In Artemis-deficient patients, cells cannot repair this damage → exaggerated toxicity. Therefore: use reduced-intensity conditioning with non-alkylating agents or carefully dose-reduced busulfan with pharmacokinetic monitoring.

Parameter	Timing	Purpose
Engraftment (ANC > 500/µL for 3 consecutive days)	Day +14 to +28	Confirms donor HSCs have engrafted and are producing neutrophils
Chimerism analysis (donor vs. recipient DNA in blood lineages)	Monthly initially → q3–6 months	Tracks % donor cells in T-cell, B-cell, myeloid, and NK-cell compartments
T-cell immune reconstitution	Monthly lymphocyte subsets (CD3/4/8, naïve T cells)	CD4⁺ > 200/µL with adequate naïve (CD45RA⁺) T cells indicates thymic function is recovering
B-cell immune reconstitution	IgG levels, specific antibody responses to vaccines (post-reconstitution)	May take 1–2 years. Patients may need IVIG until IgG production is self-sufficient
GvHD surveillance	Continuous	Skin (rash), liver (LFT), gut (diarrhoea) — graded I–IV. Treat with immunosuppression (steroids, cyclosporin, etc.)
Infection monitoring	Weekly CMV, EBV, adenovirus PCR; clinical surveillance	Reactivation is common during immune reconstitution; pre-emptive treatment
Re-vaccination	Starting 6–12 months post-HSCT when immune reconstitution confirmed	Inactivated vaccines first; live vaccines (MMR, varicella) only after confirmed T-cell reconstitution (usually > 2 years post-HSCT)

Factor	Outcome
MSD HSCT, < 3.5 months, no active infection	> 95% survival
Any donor, < 3.5 months, no active infection	> 90% survival
Any donor, with active infection at time of HSCT	50–70% survival
Delayed HSCT (> 6 months) with established infections	< 50% survival (depending on severity)

The message is clear: delayed treatment associated with complications such as bronchiectasis ^[5] — and worse, delayed treatment is associated with death.

4.2 Gene Therapy

Gene therapy represents an increasingly viable curative alternative to HSCT, particularly when no matched donor is available.

Harvest the infant's own CD34⁺ haematopoietic stem cells (from bone marrow or mobilised peripheral blood)
Transduce them ex vivo with a viral vector carrying a functional copy of the defective gene
Reinfuse the corrected autologous cells ± minimal conditioning
Corrected cells engraft and produce functional lymphocytes

The major advantage: no risk of GvHD (because the cells are the patient's own).

SCID Subtype	Vector	Status	Notes
ADA-SCID	Lentiviral vector (Strimvelis — retroviral, earlier generation; OTL-101 / EF1α-ADA lentiviral — newer)	Strimvelis approved (EU, 2016) — first gene therapy approved for PID. Newer lentiviral products in clinical trials / post-market.	ADA-SCID is the best-studied gene therapy target. Survival > 90%. Low insertion mutagenesis risk with lentiviral vectors.
X-linked SCID (IL2RG)	Lentiviral vector (self-inactivating, SIN)	Clinical trials completed with excellent results — regulatory approval expected/emerging	Early retroviral trials (2000s) caused T-cell leukaemia in ~25% due to insertional mutagenesis near LMO2 oncogene (γ-retroviral vector). Modern SIN-lentiviral vectors have a much safer integration profile.
Artemis SCID (DCLRE1C)	Lentiviral vector	Clinical trials ongoing	Important because Artemis SCID patients have radiation sensitivity → conditioning for HSCT is difficult. Gene therapy offers an alternative.
RAG1/RAG2 SCID	In preclinical development	Not yet in clinical trials as of 2025	RAG genes are large and expression is tightly regulated → technically challenging to deliver and express correctly.

Indication	Rationale
No HLA-matched sibling donor	Avoids risks of mismatched allogeneic HSCT (GvHD, graft failure)
ADA-SCID (preferred by some centres as primary treatment)	Excellent gene therapy outcomes; avoids donor search delays
Artemis SCID	Avoid toxic conditioning required for HSCT in a radiation-sensitive patient
Previous failed HSCT	Gene therapy offers a second chance at immune reconstitution

Factor	Detail
Active, uncontrolled infection	Must stabilise infections before proceeding (same as HSCT)
Subtype with no available vector	Gene therapy not available for all subtypes (e.g., RAG1/2, reticular dysgenesis)
Insertional mutagenesis risk	Early γ-retroviral vectors caused leukaemia. Modern SIN-lentiviral vectors are much safer but long-term surveillance is still required
Cost and availability	Gene therapy is extremely expensive and only available in specialised centres (not yet routinely available in Hong Kong as of 2025)

ERT is specific to ADA-SCID and serves as a bridge to definitive treatment (HSCT or gene therapy), or as long-term treatment if neither is feasible.

Feature	Detail
Drug	PEG-ADA (pegylated adenosine deaminase) — brand names: Adagen (pegademase bovine, older), Revcovi (elapegademase, newer — approved 2018)
Mechanism	Provides exogenous ADA enzyme → metabolises the toxic deoxyadenosine accumulating in plasma and cells → reduces dATP levels → allows partial lymphocyte recovery
Route	Intramuscular (IM) injection
Dose	Elapegademase: 0.2 mg/kg IM weekly (adjusted based on trough ADA levels and dAXP metabolites)
Onset of effect	Partial immune reconstitution over weeks to months (T-cell count begins to rise within 2–4 weeks)
Limitations	Does NOT fully reconstitute immunity — most patients remain partially immunodeficient. Neutralising antibodies can develop against the enzyme → reduced efficacy. Not a long-term cure — definitive treatment (HSCT or gene therapy) is still needed in most cases.
Clinical role	Bridge therapy: start immediately upon ADA-SCID diagnosis → stabilise the infant → proceed to HSCT or gene therapy. Some patients who cannot undergo HSCT/GT may remain on lifelong ERT, but long-term outcomes are inferior.

Why ERT Only Works for ADA-SCID

ADA-SCID is unique among SCID subtypes because the pathology is due to a toxic metabolite (deoxyadenosine → dATP) rather than a signalling defect. By providing the missing enzyme exogenously, you can clear the toxic metabolite and partially restore lymphocyte survival. In other SCID subtypes (e.g., IL2RG, RAG1/2), the defect is in a signalling pathway or recombination machinery that cannot be replaced by an exogenous enzyme — you must fix the gene or replace the cells.

This is NOT standard treatment for SCID, but is mentioned for completeness and to distinguish management approaches:

Complete DiGeorge syndrome (athymia, not a SCID gene defect) → treated with cultured thymus tissue transplant (not HSCT, because the problem is the thymic microenvironment, not the HSCs)
SCID patients have normal thymic epithelium (the problem is the lymphocyte precursors) → HSCT provides new precursors that can use whatever thymic remnant exists

Subtype	Definitive Treatment	Special Considerations
X-linked SCID (IL2RG)	HSCT ^[4]^[6]; gene therapy (emerging)	MSD HSCT can be done without conditioning. Haploidentical with TCRαβ depletion if no MSD. Gene therapy is increasingly available.
JAK3 SCID	HSCT	Phenocopy of X-linked SCID. Same management approach.
IL-7Rα SCID	HSCT	Usually requires some conditioning for full B-cell reconstitution
RAG1/RAG2 SCID	HSCT (usually with conditioning)	T⁻B⁻ → need both T and B cell reconstitution → conditioning improves B-cell engraftment. Watch for Omenn phenotype.
Artemis SCID	HSCT (RIC only) or gene therapy	Radiation-sensitive → avoid alkylating agents if possible; use reduced-intensity / non-genotoxic conditioning. Gene therapy trials ongoing.
ADA-SCID	PEG-ADA ERT (bridge) → HSCT or gene therapy	Start ERT immediately. Gene therapy (Strimvelis or lentiviral) is an excellent option and may be preferred over mismatched HSCT. MSD HSCT is still best if available. Must stop ERT before HSCT/GT (to remove survival advantage of uncorrected cells and allow selective advantage for corrected cells).
Reticular dysgenesis (AK2)	HSCT (MAC required)	Need myeloid AND lymphoid reconstitution → full myeloablative conditioning is necessary. No gene therapy available.

6. Family-Centred Care and Genetic Counselling

Aspect	Approach
Breaking the diagnosis	Honest, empathetic explanation: "Your baby has a condition where the immune system did not develop properly. Without treatment, the body cannot fight infections. The good news is that it IS treatable — bone marrow transplant can be curative."
Explaining urgency	"Time is critical. The sooner we transplant, the better the outcome. We need to find a donor and keep your baby safe from infections in the meantime."
Explaining isolation	Parents often struggle with isolation requirements. Acknowledge the emotional burden: "We know this is incredibly hard. But every contact with the outside world is a risk right now."
Consent for HSCT	Detailed informed consent covering: conditioning toxicity, GvHD risk, infection risk, transplant-related mortality, long-term follow-up needs. Both parents should be involved.

Inheritance	Counselling Points
X-linked recessive (IL2RG, WAS)	Mother is a carrier (usually). 50% of her sons will be affected; 50% of her daughters will be carriers. Offer carrier testing to maternal female relatives.
Autosomal recessive (ADA, RAG1/2, JAK3, IL-7Rα, Artemis)	Both parents are carriers. 25% recurrence risk for each pregnancy.
Prenatal diagnosis	Available by chorionic villus sampling (CVS) at 10–12 weeks or amniocentesis at 15–18 weeks if the family mutation is known.
Preimplantation genetic testing (PGT-M)	IVF with genetic testing of embryos before implantation. Available for families with known SCID mutations. Also allows HLA matching of embryo → potential future MSD cord blood donor for the affected child.

Concern	Monitoring
Growth and development	Plot on growth charts at every visit. Conditioning chemotherapy can affect growth.
Endocrine toxicity	Busulfan-based conditioning can cause gonadal failure, thyroid dysfunction, growth hormone deficiency → annual endocrine screening
Neurodevelopmental	Chronic illness, prolonged hospitalisation, and social isolation can impair development → developmental assessment, early intervention services
Re-vaccination programme	Begin 6–12 months post-HSCT: inactivated vaccines (DTaP, IPV, Hib, PCV, HBV). Live vaccines (MMR, varicella) only after confirmed T-cell reconstitution (usually ≥ 2 years, with lymphocyte subsets showing adequate CD4 count and naïve T cells).
Late immune reconstitution	Some patients have incomplete B-cell reconstitution → lifelong IVIG. Monitor IgG levels and vaccine responses annually.

Phase	Actions	Key Agents / Interventions
Immediate (hours)	Protective isolation; antimicrobial prophylaxis; blood product safety; NO live vaccines	Co-trimoxazole, fluconazole, aciclovir; irradiated + CMV-neg blood; isolation
Short-term (days–weeks)	IVIG replacement; treat active infections; nutritional support; HLA typing of family; donor search	IVIG 0.4–0.6 g/kg q3–4 wk; targeted antimicrobials; PEG-ADA if ADA-SCID
Definitive (weeks–months)	HSCT ^[4]^[6] (or gene therapy / ERT)	MSD > MUD > haploidentical; ± conditioning
Post-transplant (months–years)	Engraftment monitoring; GvHD management; infection surveillance; re-vaccination; growth/development	Immunosuppression (if GvHD); CMV/EBV/adeno PCR; inactivated vaccines → live vaccines
Lifelong	Long-term immune monitoring; IVIG if needed; endocrine screening; genetic counselling for family	Annual lymphocyte subsets, Ig levels, vaccine responses

High Yield Summary — Management of SCID

SCID is fatal without treatment ^[1]. Treatment: HSCT ^[4]^[6]. HSC transplantation for Severe combined immunodeficiency ^[5].
Supportive care (start IMMEDIATELY): Protective isolation, co-trimoxazole (PJP prophylaxis), fluconazole (antifungal), IVIG replacement (0.4–0.6 g/kg q3–4 wk), irradiated cellular blood components ^[12] + CMV-negative, NO live vaccines.
HSCT donor hierarchy: MSD (best, > 95% survival) > MUD > haploidentical parent > cord blood. Best outcomes when transplanted < 3.5 months of age, before infections.
Conditioning: None/minimal for MSD in T⁻B⁺ SCID; RIC for most others; MAC for T⁻B⁻ subtypes. Artemis SCID = radiation-sensitive → RIC only.
ADA-SCID: Start PEG-ADA enzyme replacement immediately as bridge; gene therapy (Strimvelis / lentiviral) is an excellent definitive option.
Gene therapy: Available for ADA-SCID (approved), X-linked SCID (emerging), Artemis SCID (trials). No GvHD risk. Long-term insertional mutagenesis surveillance required.
Delayed treatment associated with complications such as bronchiectasis ^[5]. Early treatment saves lives.
Blood products in SCID: Use irradiated cellular blood components to prevent TA-GvHD in patients with congenital immunodeficiency ^[12]. In emergency without irradiated blood: use the oldest bag of blood (> 14 days, lymphocytes should have died) ^[13].
Post-HSCT: Monitor engraftment, chimerism, T/B cell reconstitution, GvHD, infections. Re-vaccinate starting 6–12 months post-HSCT (inactivated first, live only after confirmed immune reconstitution).

Active Recall - SCID Management

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.410, SCID — fatal without treatment, clinical features) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.638, Features suggestive of PID — warning signs, complications from live vaccine) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (SCID — treatment HSCT, pathogens: PJP, BCG, Candida, viruses; ADA-SCID ERT; gene therapy) ^[5] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.9, HSC transplantation for SCID, delayed treatment → bronchiectasis, molecular basis single gene disorders) ^[6] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.18, SCID — treatment HSCT, IL2RG, tests) ^[12] Senior notes: Block A - Fever after a blood transfusion_ transfusion and related problems.pdf (p.21, Irradiated blood products for patients with congenital immunodeficiency, TA-GvHD prevention) ^[13] Senior notes: Block A - Fever after a blood transfusion_ transfusion and related problems.pdf (p.22, Emergency: use oldest bag of blood if irradiated not available)

Complications of SCID arise from two fundamentally different sources:

Complications of the disease itself (untreated SCID) — the consequences of having no functional adaptive immune system
Complications of treatment (HSCT, gene therapy, ERT) — the iatrogenic price of curing the disease

Both categories are high-yield for exams. We will work through each systematically, always linking back to the underlying mechanism.

1. Complications of Untreated / Delayed-Treatment SCID

SCID is fatal without treatment ^[1]^[2]. If not diagnosed and treated promptly, the infant accumulates progressive, irreversible end-organ damage from recurrent infections. Delayed treatment associated with complications such as bronchiectasis ^[5].

1.1 Infectious Complications (The Primary Cause of Morbidity and Mortality)

The central problem is absent adaptive immunity (no functional T cells ± no functional B cells). This means the infant cannot clear any pathogen that requires T-cell–mediated or antibody-mediated immunity — which is essentially all pathogens except those handled purely by innate immunity.

Complication	Pathophysiology	Pathogens	Clinical Features
Pneumocystis jirovecii pneumonia (PJP)	PJP requires CD4⁺ T cells for clearance. In SCID, absent CD4⁺ T cells → uncontrolled PJP proliferation in alveoli → diffuse alveolar damage	Pneumocystis jirovecii ^[4]	Tachypnoea, hypoxia, dry cough. CXR: bilateral perihilar GGO. Often the presenting infection of SCID.
Viral pneumonitis / bronchiolitis	Absent cytotoxic CD8⁺ T cells → unable to kill virus-infected respiratory epithelial cells → uncontrolled viral replication → progressive lung destruction	RSV, parainfluenza, adenovirus, influenza, human metapneumovirus	Severe, protracted course. Does not respond to supportive care alone. May progress to respiratory failure.
Bronchiectasis	Delayed treatment associated with complications such as bronchiectasis ^[5]. Recurrent/chronic lower respiratory infections → chronic airway inflammation → destruction of bronchial wall elastic tissue and cartilage → irreversible permanent bronchial dilatation	Any respiratory pathogen (bacterial superinfection on viral-damaged airways)	Chronic cough, purulent sputum, recurrent pneumonias. Once established, bronchiectasis is permanent — even after immune reconstitution by HSCT, the structural lung damage persists. This is the classic long-term complication of delayed PID diagnosis.
Chronic lung disease / pulmonary fibrosis	End-stage of chronic lung infection and inflammation	—	Restrictive lung disease, oxygen dependence

High Yield — Bronchiectasis as the Sentinel Complication of Delayed Diagnosis

Delayed treatment associated with complications such as bronchiectasis ^[5]. This point is made explicitly on the lecture slides because it encapsulates why early diagnosis matters. Bronchiectasis is irreversible — once the bronchial walls are destroyed, they do not regenerate even after successful HSCT. A child diagnosed with SCID at 12 months (after multiple pneumonias) may survive HSCT but live with lifelong bronchiectasis. A child diagnosed at 2 weeks by newborn screening and transplanted at 2 months may have completely normal lungs. This is the strongest argument for TREC-based newborn screening.

Complication	Pathophysiology	Pathogens	Clinical Features
Chronic protracted diarrhoea	Absent mucosal T-cell immunity → inability to clear enteric pathogens → chronic villous atrophy and inflammation → malabsorption	Rotavirus, norovirus, adenovirus, Cryptosporidium, Giardia	Watery or bloody diarrhoea, wasting, electrolyte disturbance
Malabsorption and failure to thrive	Direct consequence of chronic diarrhoea + villous atrophy → impaired nutrient absorption → protein-energy malnutrition	—	Weight loss / failure to gain weight, hypoalbuminaemia, micronutrient deficiencies, peripheral oedema
Cryptosporidium cholangiopathy	Cryptosporidium infects biliary epithelium → sclerosing cholangitis → biliary obstruction → secondary biliary cirrhosis. Requires T cells (specifically CD4⁺) to clear.	Cryptosporidium parvum/hominis	Cholestatic jaundice, hepatomegaly, elevated ALP/GGT. Particularly seen in Hyper-IgM syndrome but also in SCID.

Complication	Pathophysiology	Pathogens	Clinical Features
Disseminated BCG disease (BCGosis)	BCG vaccine will disseminate ^[4] in SCID. BCG is a live attenuated Mycobacterium bovis vaccine given at birth in Hong Kong. Without T-cell immunity, the organism spreads from the injection site systemically.	M. bovis BCG	Ulceration at BCG site → regional (axillary) lymphadenopathy → disseminated: hepatosplenomegaly, bone marrow involvement, miliary lung lesions, multiorgan failure. Can be fatal.
Disseminated CMV	CMV requires both CD8⁺ cytotoxic T cells and NK cells for control. In SCID (especially T⁻B⁺NK⁻), there is no defence against CMV.	CMV	Hepatitis, pneumonitis, retinitis, colitis, encephalitis. Often acquired perinatally (birth canal, breast milk).
Disseminated EBV → lymphoproliferative disease	EBV infects B cells; T cells (especially CD8⁺) normally control EBV-driven B-cell proliferation. Without T-cell surveillance → uncontrolled EBV-driven B-cell expansion	EBV	Lymphadenopathy, hepatosplenomegaly, cytopenias. Can progress to EBV-associated lymphoma even in SCID.
Disseminated adenovirus	Adenovirus clearance requires both cellular and humoral immunity	Adenovirus	Hepatitis, pneumonitis, haemorrhagic cystitis, colitis. Can be rapidly fatal.
Disseminated candidiasis	Mucocutaneous Candida defence requires Th17 cells. Deep tissue candidiasis also requires neutrophil–T-cell cooperation.	Candida albicans	Persistent oral thrush progressing to oesophageal candidiasis, candidaemia, hepatosplenic candidiasis

Complications from a live vaccine ^[2] — this is a defining feature of SCID and one of the warning signs for PID.

Vaccine	Complication in SCID	Mechanism
BCG	Disseminated BCGosis (as above)	Cannot contain attenuated mycobacterium
Rotavirus (oral live vaccine)	Chronic, protracted rotavirus gastroenteritis; vaccine-strain viraemia	Cannot clear attenuated rotavirus
OPV (oral polio vaccine)	Vaccine-associated paralytic poliomyelitis (VAPP)	Cannot clear attenuated poliovirus; reversion to neurovirulence. Note: Hong Kong uses IPV (inactivated), but travel-acquired OPV exposure is possible.
MMR / Varicella	Disseminated measles, mumps, rubella, or varicella from vaccine strains	Cannot mount immune response to attenuated live virus

1.2 Non-Infectious Complications of Untreated SCID

Mechanism: Chronic diarrhoea → malabsorption + chronic infection → catabolic state + poor caloric intake
Consequences: Growth failure (weight > height > head circumference affected), micronutrient deficiencies (zinc, iron, vitamin A, vitamin D), delayed motor milestones, muscle wasting
Paediatric significance: In infancy, FTT can affect brain growth and neurodevelopment if prolonged

Feature	Detail
Mechanism	In SCID, the infant has no T cells to reject foreign cells. Maternal T cells can cross the placenta during pregnancy and engraft in the infant (the infant's immune system cannot reject them). These maternal T cells may then recognise the infant's tissues as "foreign" (they express paternal HLA antigens) → immune attack → GvHD.
Incidence	Maternal T cells are detectable in ~40–50% of SCID infants; clinically significant GvHD occurs in a subset
Target organs	Skin (erythroderma, desquamation), liver (hepatitis, cholestasis), gut (diarrhoea, bloody stools)
Distinction from Omenn syndrome	Both can cause erythroderma and hepatosplenomegaly. Maternal GvHD: T cells have maternal HLA type on chimerism analysis. Omenn: T cells have infant's own HLA but are oligoclonal and autoreactive.
Treatment	Immunosuppression (corticosteroids, cyclosporin) as temporising measure; definitive treatment is HSCT to replace the immune system

Even in profoundly immunodeficient states, residual or dysregulated immune cells can cause autoimmune cytopenias (autoimmune haemolytic anaemia, immune thrombocytopenia), autoimmune hepatitis, or inflammatory skin disease
More common in "leaky" SCID and Omenn syndrome than in typical SCID

2. Complications of HSCT (Treatment Complications)

HSCT is curative but carries significant morbidity. Complications of HSCT ^[14] can be classified as:

Classify and name the complications of HSCT: ^[14]

Complications related to high-dose chemotherapy:

Infection

Haemorrhage

Veno-occlusive disease of liver (VOD) → chemo toxicity on liver vessels

Complications related to allogeneic HSCT:

Graft-versus-host disease (GvHD), acute or chronic

Graft rejection (i.e., host-versus-graft)

Complications related to late/long-term effects:

Cataract (mainly due to total body irradiation)

Immunodeficiency

Endocrine dysfunction and infertility

Secondary malignancy

2.2 Early Complications (< 1 year post-HSCT)

This is the most dangerous period — the infant's original (defective) immune system has been ablated by conditioning, and the new donor immune system has not yet fully developed.

Phase	Timing	Immune Defect	Pathogens	Prevention
Pre-engraftment (Day 0 to +30)	Profound neutropenia + absent T/B cells	No innate or adaptive defence	Bacteria (Pseudomonas, Gram-negatives, S. aureus), fungi (Candida, Aspergillus)	Empirical broad-spectrum antibiotics for febrile neutropenia (piperacillin-tazobactam or meropenem); antifungal prophylaxis (fluconazole or micafungin)
Early post-engraftment (Day +30 to +100)	Engrafted neutrophils, but T-cell reconstitution incomplete	Impaired cellular immunity	Viruses (CMV, EBV, adenovirus, BK virus), Pneumocystis, fungi	CMV/EBV/adenovirus PCR monitoring weekly; co-trimoxazole prophylaxis for PJP; pre-emptive antiviral therapy
Late post-engraftment (Day +100 to +365+)	Slow B-cell reconstitution ± chronic GvHD–associated immunosuppression	Impaired humoral immunity + impaired splenic function	Encapsulated bacteria (S. pneumoniae, H. influenzae), VZV reactivation	IVIG replacement until B-cell reconstitution; vaccination programme (starting 6–12 months post-HSCT)

Why CMV Is the Major Killer Post-HSCT

CMV reactivation is the single most important infectious complication in the early post-engraftment period. CMV is a latent herpesvirus present in most adults. After HSCT, the new donor immune system has not yet developed CMV-specific T cells → CMV can reactivate unchecked → pneumonitis (mortality > 50% if untreated), hepatitis, colitis, retinitis. Weekly CMV PCR surveillance with pre-emptive (val)ganciclovir is standard of care.

Acute GvHD ^[14]^[15]

Feature	Detail
Mechanism	Donor T cells in the graft recognise the recipient's tissues as "foreign" (due to minor histocompatibility antigen differences, even if HLA-matched) → immune attack on recipient organs. The opposite of graft rejection — here, it is the graft attacking the host.
Timing	Typically Day +7 to +100 post-HSCT (can occur later in some classifications)
Target organs	Skin (maculopapular rash, erythroderma), Liver (cholestatic hepatitis — elevated bilirubin, ALP, GGT), GI tract (diarrhoea, abdominal pain, bloody stools)
Grading	Grade I (skin only, < 25% BSA) to Grade IV (generalised erythroderma + severe liver/GI involvement). Grade III–IV has high mortality.
Risk factors	HLA mismatch (haploidentical > MUD > MSD), older donor, unrelated donor, inadequate GvHD prophylaxis
Prevention	GvHD prophylaxis: cyclosporin + methotrexate (traditional), or tacrolimus-based regimens. T-cell depletion of graft (TCRαβ/CD19 depletion in haploidentical HSCT). Post-transplant cyclophosphamide (PTCy) in some protocols.
Treatment	First-line: systemic corticosteroids (methylprednisolone 2 mg/kg/day). Steroid-refractory: ruxolitinib (JAK1/2 inhibitor), extracorporeal photopheresis, anti-thymocyte globulin (ATG), or other immunosuppressants.

GvHD in SCID HSCT — Special Considerations

In SCID, GvHD risk depends heavily on the donor type:

MSD with no conditioning: Very low GvHD risk (HLA-identical, minimal alloreactivity)
Haploidentical parent: High GvHD risk unless rigorous T-cell depletion is performed
MUD: Moderate risk; GvHD prophylaxis mandatory

Also note: SCID infants may already have maternal T-cell engraftment–related GvHD before HSCT (as discussed above). This must be distinguished from donor-derived acute GvHD post-HSCT.

Graft rejection (i.e., host-versus-graft) ^[14]

Feature	Detail
Mechanism	Residual host immune cells (especially NK cells, or maternally engrafted T cells) attack the donor graft → failure to engraft. Even in SCID, residual NK cells (in T⁻B⁺NK⁺ subtypes) or maternal T cells can mediate rejection.
Incidence	Higher in unconditioned transplants (because host marrow is not cleared → competitive advantage for host cells over donor cells)
Presentation	Failure of ANC to recover (ANC < 500 beyond Day +28), falling donor chimerism, pancytopenia
Management	Boost with additional donor cells, or retransplant with conditioning

Veno-occlusive disease of liver (VOD) → chemo toxicity on liver vessels ^[14]

Feature	Detail
Mechanism	Conditioning chemotherapy (especially busulfan, cyclophosphamide) damages the sinusoidal endothelium of hepatic venules → endothelial activation → microthrombosis → obstruction of hepatic venous outflow → post-sinusoidal portal hypertension
Presentation	Triad: painful hepatomegaly + fluid retention (ascites, weight gain) + jaundice (elevated bilirubin). Typically Day +10 to +21.
Risk factors in paediatrics	Young age (< 1 year), busulfan-based conditioning, pre-existing hepatic inflammation (e.g., CMV hepatitis, chronic diarrhoea–related liver damage)
Prevention	Ursodeoxycholic acid (UDCA) prophylaxis; pharmacokinetic monitoring of busulfan levels
Treatment	Supportive care (fluid restriction, diuretics). Severe VOD: defibrotide (an oligonucleotide that protects endothelium and promotes fibrinolysis).

Feature	Detail
Mechanism	Conditioning chemotherapy directly damages rapidly dividing mucosal epithelial cells → oral and GI mucositis
Oral mucositis	Painful ulceration of oral mucosa → inability to feed → requires NG tube or TPN. Risk of secondary infection (Candida, HSV).
GI mucositis	Abdominal pain, nausea, vomiting, diarrhoea
Management	Mouth care, analgesia, ice chips, IV palifermin (keratinocyte growth factor) for prevention in some protocols. TPN if unable to maintain oral intake.

Haemorrhage ^[14] — due to conditioning-induced thrombocytopenia.

Sites: GI (most common clinically significant), pulmonary, CNS (most feared), skin/mucosal
Prevention: Platelet transfusion to maintain PLT > 10–20 × 10⁹/L (higher threshold if active bleeding or procedures)
Note: Use irradiated blood products (to prevent TA-GvHD in the already immunocompromised post-HSCT state)

2.3 Late Complications (> 1 year post-HSCT)

Chronic graft-versus-host disease ^[14]^[15]

Feature	Detail
Timing	Typically > Day +100 (but can overlap with acute GvHD)
Mechanism	Chronic alloimmune response by donor immune cells against host tissues → fibrosis and autoimmune-like features
Target organs	Skin (sclerodermatous changes, lichenoid plaques), eyes (keratoconjunctivitis sicca / dry eyes), mouth (lichen planus–like oral mucositis, xerostomia), liver (cholestasis), lungs (bronchiolitis obliterans — obstructive lung disease), joints (contractures)
Significance in paediatrics	Chronic GvHD causes significant long-term morbidity in children — growth impairment, chronic pain, social isolation, school disruption. Associated immunodeficiency (both from the disease itself and from immunosuppressive treatment) → ongoing infection risk.
Treatment	Corticosteroids + calcineurin inhibitors (cyclosporin/tacrolimus). Steroid-refractory: ruxolitinib, ibrutinib, extracorporeal photopheresis, rituximab. Prolonged courses (months to years).

Bronchiolitis Obliterans — The Lung Complication of Chronic GvHD

Bronchiolitis obliterans (BO) is a fibrotic obliteration of small airways caused by chronic GvHD affecting the lungs. It presents as progressive obstructive lung disease (declining FEV1) that is often irreversible. It must be distinguished from bronchiectasis (which is due to pre-HSCT infections). Both can coexist in the same patient. BO is diagnosed by PFTs showing obstructive pattern + HRCT showing air trapping/mosaic attenuation + exclusion of infection.

Immunodeficiency (late effect of HSCT) ^[14]

Feature	Detail
B-cell reconstitution	Often the slowest lineage to recover. Many SCID patients who received unconditioned MSD HSCT have poor B-cell engraftment — donor T cells engraft (thymus-dependent, can use the patient's thymic remnant), but donor B-cell stem cells may not engraft without myeloablative conditioning clearing marrow niches. These patients remain antibody-deficient and require lifelong IVIG.
T-cell reconstitution	Usually good after successful HSCT, but may be slower in older infants (thymic involution with age) or those with chronic GvHD (thymic damage from GvHD or its treatment). Monitor naïve T-cell (CD45RA⁺) recovery as a marker of thymic function.
Functional splenic impairment	Chronic GvHD can cause functional asplenia → risk of overwhelming infection with encapsulated organisms (similar to post-splenectomy state).
Management	Lifelong monitoring of lymphocyte subsets and Ig levels. IVIG replacement if IgG production insufficient. Re-vaccination programme (starting 6–12 months post-HSCT) with monitoring of vaccine responses.

Endocrine dysfunction and infertility ^[14]^[15]

Complication	Mechanism	Paediatric Impact
Growth failure	Conditioning chemotherapy (especially busulfan, TBI) damages the growth plates and hypothalamic-pituitary axis. Chronic GvHD and its treatment (corticosteroids) further impair growth.	Short stature. Must monitor growth velocity on paediatric growth charts at every visit. May require growth hormone (GH) therapy.
Hypothyroidism	TBI or busulfan damages thyroid directly or causes hypothalamic-pituitary hypothyroidism	Annual TSH/fT4 screening. Levothyroxine replacement if needed.
Gonadal failure	Alkylating agents (busulfan, cyclophosphamide) are gonadotoxic → premature ovarian failure in girls, azoospermia in boys	May not manifest until puberty. Discuss fertility preservation options with family (oocyte/sperm cryopreservation in older children; gonadal tissue cryopreservation is experimental in infants).
Adrenal insufficiency	Prolonged corticosteroid use for GvHD → HPA axis suppression	Taper steroids slowly. Stress-dose hydrocortisone during illness if on chronic steroids.
Metabolic syndrome / T2DM	T2DM: increased 3× risk post-allogeneic HSCT ^[15]. Calcineurin inhibitors (tacrolimus) and corticosteroids cause insulin resistance.	Monitor fasting glucose and HbA1c annually.

Secondary malignancy ^[14]^[15]

Type	Mechanism	Timing
Post-transplant lymphoproliferative disease (PTLD)	EBV-driven B-cell proliferation in the immunosuppressed post-HSCT state. Donor-derived B cells infected with EBV proliferate unchecked because donor T-cell immunity is still immature.	Usually first 6–12 months post-HSCT
Myelodysplastic syndrome (MDS) / secondary acute leukaemia	Alkylating agent–induced DNA damage in residual host or donor haematopoietic cells → clonal haematopoietic disease	Years post-HSCT
Solid tumours (SCC of skin/oral cavity)	Chronic immunosuppression + chronic GvHD → impaired tumour surveillance; TBI-related late radiation carcinogenesis	Years to decades post-HSCT
Gene therapy–related insertional mutagenesis	Early γ-retroviral vectors integrated near oncogenes (e.g., LMO2) → T-cell acute lymphoblastic leukaemia. Occurred in ~25% of patients in early X-linked SCID gene therapy trials. Modern SIN-lentiviral vectors have much safer integration profiles, but long-term surveillance remains mandatory.	Years post–gene therapy

Historical Note — Gene Therapy and Leukaemia

The early gene therapy trials for X-linked SCID (Paris, London, 2000s) used γ-retroviral vectors that integrated preferentially near active gene promoters. Several patients developed T-cell ALL due to insertional activation of the LMO2 proto-oncogene. This was a major setback for the field. Modern self-inactivating (SIN) lentiviral vectors have a more random integration profile and include insulators that reduce oncogene activation risk. As of 2025, no insertional mutagenesis events have been reported with SIN-lentiviral vectors for SCID, but lifelong surveillance (annual CBP, flow cytometry if abnormal) remains standard.

Cataract (mainly due to total body irradiation) ^[14]

Mechanism: TBI causes direct radiation damage to the lens epithelium → posterior subcapsular cataracts
In paediatric SCID HSCT, TBI is rarely used (busulfan-based conditioning is preferred) → cataract risk is lower than in adult HSCT
However, prolonged corticosteroid use for chronic GvHD can also cause posterior subcapsular cataracts
Monitoring: Annual ophthalmology review

Complication	Mechanism
Neutralising anti-ADA antibodies	The immune system (as it partially reconstitutes on ERT) may produce antibodies against the exogenous PEG-ADA enzyme → decreased efficacy → worsening lymphopenia and metabolic derangement
Incomplete immune reconstitution	ERT only partially clears toxic metabolites; T-cell and B-cell recovery is incomplete compared to HSCT or gene therapy → persistent susceptibility to infections
Autoimmune phenomena	Partial immune reconstitution can lead to immune dysregulation → autoimmune haemolytic anaemia, ITP, autoimmune thyroiditis (the newly developing immune cells may be poorly tolerised)

Timeline	Complications
Neonatal / Pre-diagnosis (0–3 months)	Disseminated BCGosis (post–BCG vaccination), maternal T-cell GvHD, early opportunistic infections (PJP, Candida)
Infancy / Delayed diagnosis (3–12 months)	Chronic diarrhoea + FTT, protracted viral infections, bronchiectasis ^[5], disseminated CMV/EBV, vaccine-associated disease
Peri-HSCT (conditioning → Day +30)	Mucositis, VOD/SOS, haemorrhage, bacterial/fungal sepsis (neutropenic phase)
Early post-HSCT (Day +30 → +100)	Viral reactivation (CMV, EBV, adenovirus), acute GvHD, graft failure
Late post-HSCT (Day +100 → years)	Chronic GvHD, bronchiolitis obliterans, incomplete immune reconstitution, endocrine dysfunction, growth failure, secondary malignancy, cataracts
Lifelong	Need for IVIG (if B-cell reconstitution incomplete), infertility, psychosocial impact, second malignancy surveillance

Complication	Prevention / Surveillance
Bronchiectasis	Early diagnosis + early HSCT ^[5] — newborn screening is the best prevention
Disseminated BCG	Delay BCG vaccination until SCID is excluded (argument for universal NBS). In affected infants: multi-drug anti-TB treatment.
GvHD	HLA-matched donor selection; T-cell depletion (haploidentical); GvHD prophylaxis (cyclosporin ± MTX); post-transplant cyclophosphamide
VOD/SOS	Busulfan pharmacokinetic monitoring; ursodeoxycholic acid prophylaxis
CMV reactivation	Weekly CMV PCR monitoring; pre-emptive (val)ganciclovir
EBV-PTLD	Weekly EBV PCR monitoring; pre-emptive rituximab (anti-CD20) if EBV viraemia rises
Endocrine dysfunction	Annual TSH, fT4, fasting glucose, growth velocity. Pubertal staging from age 8–9 years.
Secondary malignancy	Annual CBP with differential; clinical exam for lymphadenopathy; ophthalmology for cataracts; dermatology for skin cancers
Psychosocial	Developmental assessment, school support, psychological support for child and family

Issue	Detail
Growth	Children are growing — endocrine toxicity from conditioning has a proportionally larger impact than in adults. Plot height, weight, and head circumference on age-specific growth charts at every visit.
Neurodevelopment	Prolonged hospitalisation, isolation, chronic illness, and pain can impair cognitive and social development. Early intervention services (physiotherapy, speech therapy, occupational therapy) should be involved from diagnosis.
Schooling	School-aged children post-HSCT may have prolonged absences → arrange hospital schooling or home tutoring.
Transition to adult care	SCID survivors who reach adolescence need structured transition to adult immunology/haematology services. Issues include: ongoing IVIG if needed, fertility counselling, career guidance, life insurance.
Fertility	Discuss gonadal toxicity with parents at the time of HSCT consent. In adolescents: offer sperm/oocyte cryopreservation before conditioning if feasible. In infants: gonadal tissue cryopreservation is experimental but available in some centres.

High Yield Summary — Complications of SCID

Complications of the disease (untreated/delayed SCID):

Fatal infections — PJP, disseminated CMV, BCGosis, viral pneumonitis, chronic diarrhoea → SCID is fatal without treatment ^[1]
Bronchiectasis — Delayed treatment associated with complications such as bronchiectasis ^[5] — irreversible structural lung damage from recurrent infections
Failure to thrive — chronic infection + malabsorption
Maternal T-cell GvHD — engrafted maternal T cells attack infant tissues (skin, liver, gut)
Live vaccine complications — BCG vaccine will disseminate ^[4]; rotavirus, OPV, MMR/varicella can all disseminate

Complications of HSCT treatment: ^[14]^[15]

Early: Infections (bacterial, fungal during neutropenia; viral reactivation during immune reconstitution), acute GvHD (skin, liver, gut), VOD/SOS, haemorrhage, mucositis, graft rejection
Late: Chronic GvHD (skin, lungs/bronchiolitis obliterans, liver, eyes), incomplete immune reconstitution (may need lifelong IVIG), endocrine dysfunction and infertility, secondary malignancy (PTLD, MDS, solid tumours), cataracts

Gene therapy complications: Insertional mutagenesis (T-ALL in early γ-retroviral trials; much safer with modern SIN-lentiviral vectors but lifelong surveillance needed)

Active Recall - Complications of SCID

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.410, SCID — fatal without treatment, clinical features) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.638, Features suggestive of PID — complications from live vaccine, warning signs) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (SCID — pathogens: PJP, BCG will disseminate, Candida, viruses; treatment HSCT) ^[5] Lecture slides: Investigations of Imm Disorders 2025.pdf (p.9, Delayed treatment associated with complications such as bronchiectasis; HSC transplantation for SCID) ^[6] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p.18, SCID — treatment HSCT, lymphopenia < 2500/ml) ^[14] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.34, Complications of HSCT — classification: chemotherapy-related, allogeneic-related, late effects) ^[15] Senior notes: Ryan Ho Haemtology.pdf (p.156, Complications and prognosis of HSCT — early and late complications, VOD, GvHD, secondary malignancy, endocrine dysfunction)

High Yield Summary

SCID is a heterogeneous group of disorders with impaired B and T cell development — it is fatal without treatment ^[1]^[2]
Most common cause: IL2RG (X-linked, loss of function) — incidence ~1 in 25,000–30,000 ^[4]
Typical presentation: Infant < 6 months with FTT, recurrent severe infections, chronic diarrhoea ^[1]^[2]
Susceptible pathogens: All — fungus (PJP), BCG vaccine will disseminate, Candida, viruses ^[4]
Key investigations: Low ALC (< 2,500/µL in infants) ^[1]^[2], absent thymus shadow on CXR ^[1]^[2], low TREC on newborn screening ^[4], low/zero T-cell count ^[4]
Definitive treatment: HSCT (haematopoietic stem cell transplantation) ^[4]^[5] — best outcomes if done early, before infections
Several PIDs are X-linked → male preponderance ^[5]
Delayed treatment is associated with complications such as bronchiectasis ^[5]
Lymphopenia in infants: < 2,500 cells/µL is abnormal — always use paediatric reference ranges ^[2]
Avoid all live vaccines in SCID — especially BCG (given at birth in HK)

High Yield Summary — Differential Diagnosis of SCID

The key differential is between categories of PID: SCID (combined T+B deficiency) vs. predominantly antibody deficiency (XLA, CVID) vs. phagocyte defect (CGD) vs. syndromic CID (DiGeorge, WAS). The discriminator is T-cell count and function.
SCID: absent/very low T cells, low ALC (< 2500/µL in infants), absent thymic shadow, susceptible to ALL pathogens ^[1]^[2]^[4]^[6]
DiGeorge: Partial T-cell deficiency + cardiac anomaly + hypocalcaemia + 22q11.2 deletion
WAS: Eczema + micro-thrombocytopenia + immunodeficiency + bloody diarrhoea ^[4]
XLA: ↓B cells, pan-hypogammaglobulinaemia, normal T cells, absent tonsils, encapsulated bacteria ^[1]
CGD: Normal CBC, normal/elevated Ig, catalase-positive organisms, granulomas ^[3]
Always exclude secondary causes: HIV (check maternal status), infant leukaemia (blood film), aplastic anaemia (pancytopenia), medication effects
Omenn syndrome: "Leaky SCID" with oligoclonal T cells, erythroderma, eosinophilia, ↑IgE — a SCID variant, not a separate entity

High Yield Summary — Diagnosis of SCID

Screening: Low TREC on NBS ^[4]^[6] (or clinical suspicion in non-NBS settings) + ALC < 2500/µL in infants ^[2] + absent thymic shadow on CXR ^[1]^[2]
Confirmation: Flow cytometry showing low/zero T-cell count ^[4]^[6], low naïve T cells ^[4]; absent T-cell proliferation to PHA; low immunoglobulins (especially IgA and IgM)
Classification: T/B/NK phenotype narrows the genetic differential — T⁻B⁺NK⁻ → IL2RG (X-linked, most common) ^[4]^[6]
Genetic diagnosis: Targeted panel or WES confirms specific mutation → guides treatment selection
Many PIDs are treatable — HSC transplantation for SCID. Delayed treatment associated with complications such as bronchiectasis ^[5]
Paediatric pitfalls: Always use age-appropriate ALC ranges; neonatal IgG is maternal; naïve T cells distinguish SCID from maternal engraftment and Omenn syndrome; in Omenn/SCID, T cells may be maternally grafted memory T cells ^[11]

High Yield Summary — Management of SCID

SCID is fatal without treatment ^[1]. Treatment: HSCT ^[4]^[6]. HSC transplantation for Severe combined immunodeficiency ^[5].
Supportive care (start IMMEDIATELY): Protective isolation, co-trimoxazole (PJP prophylaxis), fluconazole (antifungal), IVIG replacement (0.4–0.6 g/kg q3–4 wk), irradiated cellular blood components ^[12] + CMV-negative, NO live vaccines.
HSCT donor hierarchy: MSD (best, > 95% survival) > MUD > haploidentical parent > cord blood. Best outcomes when transplanted < 3.5 months of age, before infections.
Conditioning: None/minimal for MSD in T⁻B⁺ SCID; RIC for most others; MAC for T⁻B⁻ subtypes. Artemis SCID = radiation-sensitive → RIC only.
ADA-SCID: Start PEG-ADA enzyme replacement immediately as bridge; gene therapy (Strimvelis / lentiviral) is an excellent definitive option.
Gene therapy: Available for ADA-SCID (approved), X-linked SCID (emerging), Artemis SCID (trials). No GvHD risk. Long-term insertional mutagenesis surveillance required.
Delayed treatment associated with complications such as bronchiectasis ^[5]. Early treatment saves lives.
Blood products in SCID: Use irradiated cellular blood components to prevent TA-GvHD in patients with congenital immunodeficiency ^[12]. In emergency without irradiated blood: use the oldest bag of blood (> 14 days, lymphocytes should have died) ^[13].
Post-HSCT: Monitor engraftment, chimerism, T/B cell reconstitution, GvHD, infections. Re-vaccinate starting 6–12 months post-HSCT (inactivated first, live only after confirmed immune reconstitution).

High Yield Summary — Complications of SCID

Complications of the disease (untreated/delayed SCID):

Fatal infections — PJP, disseminated CMV, BCGosis, viral pneumonitis, chronic diarrhoea → SCID is fatal without treatment ^[1]
Bronchiectasis — Delayed treatment associated with complications such as bronchiectasis ^[5] — irreversible structural lung damage from recurrent infections
Failure to thrive — chronic infection + malabsorption
Maternal T-cell GvHD — engrafted maternal T cells attack infant tissues (skin, liver, gut)
Live vaccine complications — BCG vaccine will disseminate ^[4]; rotavirus, OPV, MMR/varicella can all disseminate

Complications of HSCT treatment: ^[14]^[15]

Early: Infections (bacterial, fungal during neutropenia; viral reactivation during immune reconstitution), acute GvHD (skin, liver, gut), VOD/SOS, haemorrhage, mucositis, graft rejection
Late: Chronic GvHD (skin, lungs/bronchiolitis obliterans, liver, eyes), incomplete immune reconstitution (may need lifelong IVIG), endocrine dysfunction and infertility, secondary malignancy (PTLD, MDS, solid tumours), cataracts

Gene therapy complications: Insertional mutagenesis (T-ALL in early γ-retroviral trials; much safer with modern SIN-lentiviral vectors but lifelong surveillance needed)

Severe Combined Immunodeficiency

Severe Combined Immunodeficiency (SCID) — Paediatrics

1. Definition

2. Epidemiology

Incidence

Sex Distribution

Age at Presentation

Hong Kong / Asian Context

3. Anatomy and Function: The Immune System Relevant to SCID

3.1 T-Cell Development (Thymus-Dependent)

3.2 B-Cell Development (Bone Marrow → Periphery)

3.3 NK-Cell Development

3.4 The Thymus in SCID

4. Aetiology and Pathophysiology

4.1 Classification by Lymphocyte Phenotype

4.2 Detailed Pathophysiology of Major Subtypes

A. X-Linked SCID (IL2RG / γc Deficiency) — T⁻B⁺NK⁻

B. JAK3 Deficiency — T⁻B⁺NK⁻

C. IL-7Rα Deficiency — T⁻B⁺NK⁺

D. RAG1/RAG2 Deficiency — T⁻B⁻NK⁺

E. Artemis (DCLRE1C) / DNA Ligase IV / NHEJ1 Deficiency — T⁻B⁻NK⁺

F. Adenosine Deaminase (ADA) Deficiency — T⁻B⁻NK⁻

G. Reticular Dysgenesis (AK2 Deficiency) — T⁻B⁻NK⁻ + Agranulocytosis

4.3 Summary Diagram of Pathophysiology

5. Classification

5.1 By Immunophenotype (Most Clinically Useful)

5.2 By the IUIS (International Union of Immunological Societies) Classification [6]

5.3 By Inheritance Pattern

5.4 Typical vs. Leaky (Atypical) SCID

6. Clinical Features

6.1 General Presentation Pattern

6.2 Symptoms (with Pathophysiological Basis)

A. Recurrent and Severe Infections

B. Failure to Thrive (FTT)

C. Chronic Diarrhoea

D. Skin Manifestations

6.3 Signs (with Pathophysiological Basis)

A. Absent Lymphoid Tissue

B. Growth Parameters

C. Signs of Opportunistic Infections

D. Lymphocyte Count

6.4 Warning Signs of PID (The "10 Warning Signs" — Jeffrey Modell Foundation, adapted for Paediatrics)

7. Investigations (Preview — to be expanded later)

8. Key Distinguishing Features from Related Conditions

9. Specific Paediatric Considerations

9.1 Age-Appropriate Physiology

9.2 Maternal IgG

9.3 Family-Centred Care and Communication

9.4 Consent and Assent

10. Treatment Overview (Preview — to be expanded later)

Active Recall - SCID (Definition, Epidemiology, Pathophysiology & Clinical Features)

References

Differential Diagnosis of Severe Combined Immunodeficiency (SCID) — Paediatrics

1. Framework: The "Recurrently Infected Infant" — Is It Immunodeficiency?

2. Differential Diagnosis of SCID Specifically

2.1 Other Primary Immunodeficiencies (PIDs) That May Mimic SCID

A. Combined Immunodeficiency with Syndromic Features (CID — Less Severe than SCID)

B. Predominantly Antibody Deficiencies (IUIS Category 3)

C. Phagocyte Defects (IUIS Category 5)

D. Complement Deficiencies (IUIS Category 8)

2.2 Secondary (Acquired) Causes of Immunodeficiency in Infants

3. Differential Diagnosis Algorithm

4. Key Discriminating Features — Summary Table

5. Differentiating SCID Subtypes from Each Other

6. Approach to the Infant with Suspected SCID — Clinical Reasoning

Step 1: Is this immunodeficiency at all?

Step 2: Primary or secondary immunodeficiency?

Step 3: Which category of PID?

Step 4: If SCID, which subtype?

7. Conditions That Can Be Confused with SCID — Pitfalls

8. Relevance to Hong Kong Paediatric Practice

Active Recall - Differential Diagnosis of SCID

References

Diagnostic Criteria, Algorithm and Investigations for Severe Combined Immunodeficiency (SCID) — Paediatrics

1. Diagnostic Criteria for SCID

1.1 PIDTC Diagnostic Criteria for SCID (2022 Updated)

A. Typical SCID

B. Leaky / Atypical SCID

C. Omenn Syndrome

1.2 Practical "Bedside" Criteria — When to Suspect SCID