Hyper-IgM syndromes are a group of primary immunodeficiency disorders, typically presenting in early childhood, characterized by defective immunoglobulin class-switch recombination resulting in normal or elevated IgM levels with markedly reduced IgG, IgA, and IgE, leading to recurrent sinopulmonary and opportunistic infections.

Hyper-IgM Syndromes (HIGM) — Paediatrics

Hyper-IgM syndromes (HIGM) are a heterogeneous group of primary immunodeficiency disorders (PIDs) — now more broadly termed inborn errors of immunity (IEI) — characterised by:

Normal or elevated serum IgM
Markedly decreased or absent IgG, IgA, and IgE
A failure of immunoglobulin class-switch recombination (CSR) and, in some subtypes, somatic hypermutation (SHM)

The name "Hyper-IgM" is actually a misnomer in some patients — the IgM is not always elevated. The hallmark is really the inability to switch from IgM to downstream isotypes (IgG, IgA, IgE). Because class switching is blocked, the B cells keep churning out IgM (which may accumulate), while IgG/IgA/IgE remain profoundly low ^[1]^[2].

Pathophysiology: failure to switch IgM to IgG/A ^[1] Ix: ↑IgM, ↓IgG/A/E ^[1]

Why is IgM elevated?

During a normal immune response, naïve B cells first produce IgM. T-cell help (via CD40L–CD40 interaction and cytokines) triggers the B cell to undergo class-switch recombination in the germinal centre, rearranging its heavy-chain constant region genes to produce IgG, IgA, or IgE. When this signalling is defective, B cells are "stuck" at the IgM stage → IgM production continues (and may increase in the face of chronic antigenic stimulation) while IgG/IgA/IgE remain absent or profoundly low.

2. Epidemiology and Risk Factors

Overall incidence of HIGM syndromes: approximately 1 in 100,000–500,000 live births (varies by subtype and population)
HIGM type 1 (CD40L deficiency / X-linked HIGM) accounts for ~65–70% of all HIGM cases and is the most common subtype
Males are predominantly affected in X-linked forms (types 1 and 5); autosomal recessive forms affect both sexes equally

Most children with X-linked HIGM present in the first 1–2 years of life, typically after 4–6 months when maternal IgG has waned
- This timing is the same principle as for other humoral immunodeficiencies: the neonate is initially protected by transplacental maternal IgG (half-life ~21 days); by 4–6 months, maternal IgG is depleted, and the infant must rely on endogenous antibody production — which is defective in HIGM

Timing: present after 4–6 months due to maternal IgG depletion ^[1]

Family history of immunodeficiency or unexplained early death (especially males in X-linked forms)
Consanguinity increases risk for autosomal recessive forms (types 2, 3, 4)
No specific ethnic or geographic predisposition, though in Hong Kong the prevalence of PID is increasing in recognition due to improved genetic testing and newborn screening awareness ^[3]

To understand HIGM syndromes, you must understand the molecular machinery of class-switch recombination (CSR) and somatic hypermutation (SHM), because each HIGM subtype results from a defect at a specific step in this pathway.

3.1 Normal Immunoglobulin Class Switching — A Primer

Class-switch recombination (CSR) is the process by which an activated B cell changes the constant region of its immunoglobulin heavy chain (e.g., from Cμ [IgM] to Cγ [IgG], Cα [IgA], or Cε [IgE]) without changing its antigen specificity (the variable region stays the same). This occurs in the germinal centres of secondary lymphoid organs.

Steps of CSR (simplified):

Key molecules involved:

CD40 ligand (CD40L / CD154) — expressed on activated T-helper cells
CD40 — expressed on B cells, macrophages, dendritic cells
NF-κB essential modulator (NEMO / IKKγ) — intracellular signalling downstream of CD40
Activation-induced cytidine deaminase (AID / AICDA) — the enzyme that initiates DNA breaks for both CSR and SHM
Uracil-DNA glycosylase (UNG) — processes AID-generated uracil residues in DNA

Somatic hypermutation (SHM) is a related process (also AID-dependent) that introduces point mutations into the variable region of the immunoglobulin gene, allowing affinity maturation (selecting B cells that bind antigen more tightly).

Key Concept — T-cell vs. B-cell Intrinsic Defects

HIGM syndromes are classified by whether the defect is:

T-cell intrinsic (defective T cell signal TO the B cell) → e.g., CD40L deficiency (Type 1), NEMO deficiency → these patients also have defective T-cell–macrophage interactions, leading to opportunistic infections (e.g., Pneumocystis jirovecii, Cryptosporidium)
B-cell intrinsic (the B cell itself cannot perform CSR/SHM) → e.g., AID deficiency (Type 2), UNG deficiency (Type 5), CD40 deficiency (Type 3) → more purely humoral phenotype (though CD40 deficiency also impairs macrophage activation)

4. Aetiology and Classification

Type	Gene Defect	Protein	Inheritance	Mechanism	CSR	SHM	Opportunistic Infections
HIGM Type 1	CD40LG (Xq26.3)	CD40 Ligand (CD154)	X-linked recessive	T-cell intrinsic: no CD40L → no T→B signal for CSR; also no T→macrophage activation	Absent	Absent	Yes (PJP, Cryptosporidium)
HIGM Type 2	AICDA (12p13)	AID (activation-induced cytidine deaminase)	Autosomal recessive	B-cell intrinsic: no AID → no DNA breaks for CSR or SHM	Absent	Absent	No
HIGM Type 3	CD40 (20q13)	CD40	Autosomal recessive	B-cell intrinsic (but also affects macrophages): no CD40 → same pathway blocked as Type 1	Absent	Absent	Yes (similar to Type 1)
HIGM Type 4	Unknown	Unknown	Autosomal recessive	B-cell intrinsic: CSR defective but SHM intact	Absent	Normal	No
HIGM Type 5	UNG (12q24.1)	Uracil-DNA glycosylase	Autosomal recessive	B-cell intrinsic: cannot process AID-induced lesions for CSR	Defective	Partially defective	No
NEMO deficiency	IKBKG (Xq28)	NF-κB essential modulator (IKKγ)	X-linked recessive	Intracellular signalling: defective NF-κB pathway downstream of CD40	Defective	Variable	Yes

Mutation in Exon 5 of CD40L: c.761C>T, P.T254M — This is a specific mutation noted in the HKU lecture for HIGM Type 1 ^[4]

4.2 Focus on the Most Important Subtypes

This is the most common and most important subtype for exams.

Genetics:

Gene: CD40LG (also called TNFSF5) on Xq26.3
Inheritance: X-linked recessive → almost exclusively affects boys
Multiple mutation types: missense, nonsense, splice-site, deletions

Pathophysiology (step by step):

CD40L is normally expressed on activated CD4+ T-helper cells
When a T cell recognises antigen presented by a B cell (via MHC-II), the T cell upregulates CD40L
CD40L binds CD40 on the B cell → triggers intracellular signalling (NF-κB, JAK3, TRAF proteins) → activates AID → CSR and SHM occur
In HIGM Type 1, CD40L is absent or non-functional → the B cell never receives the "switch" signal → remains stuck producing IgM
Critically, CD40L also signals to macrophages and dendritic cells via CD40:
- CD40L–CD40 on macrophages → activates macrophage killing of intracellular pathogens (particularly Pneumocystis, Cryptosporidium, Mycobacteria)
- CD40L–CD40 on dendritic cells → upregulates co-stimulatory molecules, enhances antigen presentation
Therefore, HIGM Type 1 is not just an antibody deficiency — it is functionally a combined immunodeficiency with impaired cellular immunity against intracellular/opportunistic pathogens

High Yield — Why Opportunistic Infections in HIGM Type 1?

The reason HIGM Type 1 patients get Pneumocystis jirovecii pneumonia (PJP) and Cryptosporidium (unlike patients with pure antibody deficiencies like XLA or CVID) is that the CD40L–CD40 axis is also critical for T cell–macrophage interaction. Without CD40L, macrophages cannot be properly activated to kill intracellular pathogens. This is the same reason HIGM Type 3 (CD40 deficiency) also causes opportunistic infections.

Gene: AICDA on chromosome 12p13
Inheritance: Autosomal recessive
Pathophysiology: AID is the enzyme that initiates DNA double-strand breaks at immunoglobulin switch regions. Without AID, neither CSR nor SHM can occur → IgM is produced but cannot switch, and antibodies cannot undergo affinity maturation
Clinical distinction from Type 1: No opportunistic infections (because T cell–macrophage signalling via CD40L–CD40 is intact) → patients have a more purely humoral phenotype
A distinctive feature: Massive lymphoid hyperplasia (enlarged tonsils, lymph nodes, germinal centres are enlarged but aberrant) — because B cells proliferate in germinal centres but cannot complete CSR

Gene: CD40 (also called TNFRSF5) on chromosome 20q13
Inheritance: Autosomal recessive
Pathophysiology: Mirrors Type 1 clinically because CD40 is the receptor for CD40L → same pathway blocked
Clinically indistinguishable from Type 1 (including opportunistic infections) but affects both sexes

Gene: IKBKG on Xq28
Inheritance: X-linked
NEMO is the regulatory subunit of the IKK complex that activates NF-κB → downstream of CD40 signalling
Patients have ectodermal dysplasia (anhidrotic: sparse hair, conical teeth, absent sweat glands) PLUS immunodeficiency with hyper-IgM phenotype
Also susceptible to opportunistic infections and mycobacterial disease

6. Clinical Features

Most HIGM patients present in infancy or early childhood (typically between 6 months and 2 years), after the decline of protective maternal IgG. The presentation depends on the specific subtype, but common features include:

Commonly cause sinopulmonary and GI infections — sites where most Ig deposits, open to external environment ^[1]

Symptom	Pathophysiological Basis
Recurrent sinopulmonary infections (otitis media, sinusitis, pneumonia)	IgG/IgA deficiency → loss of opsonisation and mucosal defence at respiratory surfaces, which are constantly exposed to environmental pathogens. Encapsulated bacteria (S. pneumoniae, H. influenzae) rely on antibody-mediated opsonisation for clearance
Recurrent/chronic diarrhoea	(1) IgA deficiency → loss of mucosal immunity in the gut; (2) In Type 1/3: Cryptosporidium parvum infection (opportunistic) → chronic watery diarrhoea → may lead to sclerosing cholangitis (see below); (3) Giardia lamblia infection (cleared by IgA)
Failure to thrive (FTT)	Chronic infections + chronic diarrhoea + malabsorption → caloric deficit → poor growth
Oral ulcers / stomatitis	Neutropenia (seen in ~50% of Type 1) → poor mucosal defence → recurrent aphthous ulcers, gingivitis
Pneumocystis jirovecii pneumonia (PJP) — often the presenting illness in Type 1	CD40L deficiency → defective macrophage activation → inability to kill intracellular pathogens. PJP classically presents with tachypnoea, dry cough, hypoxia, and bilateral ground-glass opacities on CXR. Often the first presentation in infancy
Prolonged or severe viral infections (particularly in Type 1/3)	Impaired T-cell priming via CD40–CD40L axis → suboptimal CD8+ cytotoxic T-cell responses
Jaundice, right upper quadrant pain	Cryptosporidium-associated sclerosing cholangitis → progressive biliary damage → cholestatic jaundice

Sign	Pathophysiological Basis
Lymphadenopathy (especially in Type 2 — AID deficiency)	In AID deficiency, B cells proliferate in germinal centres but cannot complete CSR → massive lymphoid hyperplasia with enlarged tonsils, lymph nodes. In Type 1, lymphoid tissue may be small (because the CD40L signal also drives germinal centre formation)
Hepatosplenomegaly	(1) Chronic infection → reactive lymphoid hyperplasia; (2) Cryptosporidium cholangitis → hepatomegaly; (3) Autoimmune cytopenias → extramedullary haematopoiesis
Oral thrush (candidiasis)	Defective T-cell–macrophage activation (Type 1/3) → susceptibility to Candida
Neutropenia (cyclical or chronic)	Seen in ~50% of HIGM Type 1 patients. Mechanism multifactorial: (1) anti-neutrophil antibodies (autoimmune), (2) impaired granulopoiesis, (3) direct marrow effect. Neutropenia contributes to oral ulcers, gingivitis, skin infections
Signs of sclerosing cholangitis (jaundice, hepatomegaly, pruritus)	Cryptosporidium colonises the biliary epithelium → chronic inflammation → fibrosis → sclerosing cholangitis. This is a major cause of morbidity and mortality in HIGM Type 1
Absence of palpable lymph nodes and small/absent tonsils (in Type 1)	Without CD40L→CD40 signalling, germinal centres cannot form properly → lymphoid hypoplasia (contrast with Type 2 where germinal centres are hyperplastic)
Ectodermal dysplasia features (in NEMO deficiency)	Conical teeth, sparse hair, anhidrosis → due to NF-κB role in ectodermal development

Clinical Pearl — Tonsil/Lymph Node Size Helps Distinguish Subtypes

HIGM Type 1 (CD40L deficiency): Tonsils and lymph nodes are small or absent (germinal centre formation requires CD40L–CD40)
HIGM Type 2 (AID deficiency): Tonsils and lymph nodes are enlarged (germinal centres form but B cells cannot complete CSR → they accumulate)
This is analogous to XLA (no B cells at all → absent tonsils) vs CVID (B cells present but dysfunctional → lymphoid tissue may be present)

Complication	Subtype Most Affected	Mechanism
Cryptosporidium sclerosing cholangitis	Type 1, Type 3	Defective macrophage activation → chronic biliary Cryptosporidium infection → progressive fibrosis → liver failure. Leading cause of death in HIGM Type 1
Autoimmune cytopenias (haemolytic anaemia, ITP, autoimmune neutropenia)	All types (especially Type 1)	Immune dysregulation — the same mechanisms that fail to produce proper IgG also disturb immune tolerance. Excess IgM may form autoantibodies
Malignancy (lymphoma, hepatocellular carcinoma, cholangiocarcinoma)	Type 1	Chronic immune stimulation + immune dysregulation → lymphoproliferation → lymphoma. Chronic cholangitis → cholangiocarcinoma/HCC
Bronchiectasis	All types	Recurrent/chronic sinopulmonary infections → airway wall damage → irreversible bronchial dilation
Chronic liver disease	Type 1, Type 3	Cryptosporidium + other hepatotropic infections

Pathogen	Type of Infection	HIGM Subtype	Why?
Pneumocystis jirovecii	PJP (pneumonia)	Type 1, 3	Defective T→macrophage activation
Cryptosporidium parvum	Chronic diarrhoea, sclerosing cholangitis	Type 1, 3	Defective T→macrophage activation; IgA deficiency
**Streptococcus pneumoniae, Haemophilus influenzae**	Sinusitis, otitis media, pneumonia	All types	IgG deficiency → loss of opsonisation
Giardia lamblia	Chronic diarrhoea	All types	IgA deficiency at mucosal surfaces
Candida spp.	Oral/systemic candidiasis	Type 1, 3	Defective Th1/macrophage responses
Enterovirus	Chronic meningoencephalitis	All types	IgG deficiency
Mycobacteria (including BCG)	Disseminated infection	Type 1, 3 (and NEMO)	Defective macrophage activation

Common pathogens (humoral deficiency): encapsulated bacteria, Giardia, enterovirus ^[1]

7. Investigations — Approach to Diagnosis

Phenotypic investigations for HIGM: expression of CD40-ligand on activated T cells ^[3] Genetic testing has an important role in: Diagnosis (All cases), Family studies (HIGM, HAE cases), Prognostication, Prenatal diagnosis ^[3]

Investigation	Expected Finding in HIGM	Rationale
Serum immunoglobulins (IgG, IgA, IgM, IgE)	↑ or normal IgM; ↓↓ IgG; ↓↓ IgA; ↓↓ IgE	The hallmark: failure to class-switch → IgM persists, other isotypes absent. Note: IgM may sometimes be normal rather than elevated
Complete blood count (CBC) with differential	Neutropenia in ~50% of Type 1; lymphocyte count usually normal (unlike SCID)	Neutropenia is a distinctive feature and contributes to morbidity
Lymphocyte subsets (flow cytometry) — T, B, NK cell counts	B cells (CD19+/CD20+) are present (unlike XLA where B cells are absent); T cells usually normal in number	HIGM is not a B-cell maturation defect (unlike XLA) — B cells are present but cannot class-switch
Vaccine antibody responses	Poor or absent IgG responses to protein and polysaccharide vaccines; may have IgM responses	Confirms functional antibody deficiency; even after vaccination, no IgG is produced

Investigation	Finding	Which Subtype
CD40L expression on activated T cells (flow cytometry)	Absent or reduced CD40L after T-cell activation (e.g., with PMA/ionomycin)	Type 1 (CD40L deficiency)
CD40 expression on B cells (flow cytometry)	Absent or reduced CD40	Type 3 (CD40 deficiency)
AID protein expression / in vitro CSR assay	Absent AID; B cells fail to undergo CSR in vitro	Type 2 (AID deficiency)
Genetic sequencing (Sanger or next-generation sequencing / whole exome sequencing)	Identifies the specific mutation in CD40LG, AICDA, CD40, UNG, IKBKG, etc.	All types — genetic testing is definitive

Cell function: e.g. expression of CD40-ligand on activated T cells for hyper-IgM syndrome ^[3]

Investigation	Purpose
Stool microscopy and PCR for Cryptosporidium	Detect Cryptosporidium (especially in Type 1/3)
Liver function tests (LFTs) + GGT	Elevated GGT/ALP suggests cholangiopathy; elevated bilirubin suggests cholestasis
Abdominal ultrasound / MRCP	Assess for sclerosing cholangitis (biliary dilation, wall thickening)
Chest X-ray / HRCT	Assess for PJP (bilateral GGO), bronchiectasis
Liver biopsy (if indicated)	Confirm sclerosing cholangitis, exclude malignancy

Feature	HIGM	XLA	CVID
IgM	↑ or normal	↓↓↓ (pan-hypogammaglobulinaemia)	↓↓
IgG	↓↓	↓↓↓	↓↓↓
IgA	↓↓	↓↓↓	↓
B cells (CD19+)	Present	Absent (< 1%)	Present
Tonsils/lymph nodes	Small (Type 1) or enlarged (Type 2)	Absent	Variable
Opportunistic infections	Yes (Type 1, 3)	No	No (usually)
Inheritance	XLR (Type 1) or AR	XLR	Variable / sporadic
Age at onset	Infancy	Infancy	Childhood–adulthood

8. Relevant Paediatric Considerations

Immunoglobulin levels vary markedly with age in paediatrics. Always compare to age-matched reference ranges:
- At birth: IgG is high (maternal transfer); IgM is low (does not cross placenta)
- By 3–6 months: "physiological trough" of IgG as maternal IgG wanes
- IgM reaches adult levels by ~1 year; IgG by ~5–7 years; IgA by ~10–16 years
Lymphocyte count: In infants, normal ALC is higher than in adults. Lymphopenia in infants is defined by < 2,500 cells/µL ^[5]; in children > 5 years, < 1,500 cells/µL

TREC (T-cell receptor excision circle) screening is used for SCID, not specifically for HIGM
KREC (kappa-deleting recombination excision circle) screening can detect B-cell deficiencies including XLA, but HIGM has normal B-cell numbers → KREC is usually normal in HIGM
HIGM is NOT reliably detected by current newborn screening programmes → clinical suspicion is key

When a child is diagnosed with HIGM, the family must be counselled on:
- The genetic basis and inheritance pattern (X-linked vs AR)
- The need for lifelong immunoglobulin replacement therapy
- Avoidance of live vaccines (especially BCG, OPV, rotavirus, varicella, MMR) — because of the combined immunodeficiency in Type 1/3
- Water safety (avoiding potentially Cryptosporidium-contaminated water)
- The possibility of haematopoietic stem cell transplantation (HSCT) as a curative option
- Genetic counselling for carrier detection and prenatal diagnosis

For young children: parental informed consent is required for all investigations and treatments
For adolescents (typically ≥ 12 years in HK): assent from the patient plus parental consent
Genetic testing requires careful pre-test counselling regarding implications for the child and family members

HKU Queen Mary Hospital has an active primary immunodeficiency programme under the Department of Paediatrics and Adolescent Medicine, led by Professor YL Lau (as referenced in lecture slides) ^[2]^[3]
Genetic testing in Hong Kong is increasingly accessible via clinical exome/genome sequencing through the Hong Kong Genome Project and hospital-based molecular diagnostic labs
BCG vaccination is given at birth in Hong Kong as part of the routine childhood immunisation schedule → in undiagnosed HIGM Type 1/3 or NEMO deficiency, disseminated BCG disease can be a presenting feature (because macrophage activation is defective → cannot contain mycobacteria)
- This is a critical Hong Kong–specific consideration: if a neonate develops disseminated BCG (regional lymphadenitis, osteitis, or systemic BCG-osis), always consider a combined immunodeficiency including HIGM Type 1

Feature	Type 1 (CD40L)	Type 2 (AID)	Type 3 (CD40)	Type 5 (UNG)	NEMO
Inheritance	XLR	AR	AR	AR	XLR
Defective molecule	CD40L (T cell)	AID (B cell)	CD40 (B cell)	UNG (B cell)	NEMO (signalling)
Lymph nodes/tonsils	Small/absent	Enlarged	Small/absent	Variable	Small
Opportunistic infections	Yes	No	Yes	No	Yes
Neutropenia	~50%	Rare	Possible	Rare	Possible
Sclerosing cholangitis	Yes	No	Yes	No	Possible
Autoimmunity	Yes	Yes	Yes	Yes	Yes
Malignancy risk	Increased	Increased	Increased	Uncertain	Increased
CSR	Absent	Absent	Absent	Defective	Defective
SHM	Absent	Absent	Absent	Partially defective	Variable
Ectodermal dysplasia	No	No	No	No	Yes

Features suggestive of PID (from lecture slides and senior notes) ^[1]^[5]:

Family history of immunodeficiency or unexplained early death (before 30)

Failure to gain weight or grow normally (failure to thrive)

Persistent lymphopenia

≥ 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

Complications from a live vaccine

≥ 2 Months of antibiotics with little effect

High Yield Summary

Hyper-IgM Syndromes — Key Points for Exams:

Definition: Group of PIDs characterised by ↑IgM, ↓↓IgG/IgA/IgE due to failure of class-switch recombination
Most common subtype: HIGM Type 1 (CD40L deficiency) — X-linked recessive, ~65–70% of cases
Core pathophysiology: CD40L–CD40 interaction is needed for (a) B-cell class switching and (b) macrophage activation → its absence causes both antibody deficiency AND susceptibility to opportunistic infections (PJP, Cryptosporidium)
Why opportunistic infections in Type 1/3 but not Type 2?: CD40L and CD40 are involved in T-cell–macrophage communication; AID is purely intrinsic to B-cell CSR
Key diagnostic test: Flow cytometry for CD40L expression on activated T cells (absent in Type 1); genetic sequencing is confirmatory for all subtypes
Hallmark Ix pattern: ↑IgM, ↓IgG/IgA/IgE, B cells present (CD19+), normal T cell counts (unlike SCID)
Major complications: Cryptosporidium sclerosing cholangitis (leading cause of death in Type 1), autoimmune cytopenias, malignancy, bronchiectasis
Neutropenia occurs in ~50% of Type 1 → oral ulcers, gingivitis
Tonsil clue: Small/absent in Type 1; enlarged in Type 2 (AID deficiency — B cells accumulate in germinal centres)
Curative treatment: HSCT (haematopoietic stem cell transplantation) — especially for Type 1/3 with opportunistic infections; IgG replacement is a lifelong supportive measure
Hong Kong relevance: BCG given at birth → disseminated BCG can be a presenting feature in undiagnosed HIGM Type 1/3/NEMO deficiency

Active Recall - Hyper-IgM Syndromes

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 410 — Primary Immunodeficiency, Hyper IgM syndrome) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (p. 1–2 — CFB + Block A/C immunodeficiency, HIGM Type 1 CD40L mutation) ^[3] Lecture slides: Investigations of Imm Disorders 2025.pdf (p. 10, 21 — Approach to Diagnosis of PID, CD40-ligand expression, genetic testing for HIGM) ^[4] Senior notes: Jerry's immunodeficiencies.pdf (p. 1 — Mutation in Exon 5 of CD40L: c.761C>T, P.T254M) ^[5] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 638 — Features suggestive of PID, lymphopenia definition in infants)

Differential Diagnosis of Hyper-IgM Syndromes

When you encounter a child with the laboratory hallmark of elevated or normal IgM alongside markedly reduced IgG, IgA, and IgE, you must work through a structured differential. The challenge is twofold: (1) distinguishing between the different HIGM subtypes themselves (covered in the prior classification section), and (2) differentiating HIGM from other conditions that share overlapping clinical or immunological features.

Think of the differential in layers:

Other primary immunodeficiencies (PIDs) that can present with similar immunoglobulin patterns or recurrent infections in infancy
Secondary (acquired) causes of hypogammaglobulinaemia with preserved or elevated IgM
Conditions that mimic the clinical presentation (recurrent sinopulmonary infections, opportunistic infections, neutropenia, lymphadenopathy)

2. Differential Diagnosis — Primary Immunodeficiencies

Feature	XLA	HIGM
IgM	↓↓↓ (all isotypes very low)	↑ or normal
IgG, IgA, IgE	↓↓↓	↓↓
B cells (CD19+)	Absent (< 1%)	Present
Gene	BTK (Bruton tyrosine kinase)	CD40LG, AICDA, CD40, etc.
Inheritance	XLR	XLR (Type 1) or AR
Tonsils	Absent	Small (Type 1) or enlarged (Type 2)
Opportunistic infections	No	Yes (Type 1, 3)

Why the confusion arises: Both XLA and HIGM present in male infants (X-linked forms) after 4–6 months with recurrent sinopulmonary infections once maternal IgG wanes. The key differentiator is the immunoglobulin pattern and B-cell count:

XLA: pan-hypogammaglobulinaemia with absent B cells — the problem is B-cell maturation arrest (no mature B cells at all) ^[1]
HIGM: B cells are present and can produce IgM, but cannot class-switch → IgM is normal/elevated while IgG/IgA/IgE are low

↓B cells, pan-hypo(γ)globulinemia (hence absent tonsil, and immunization being useless!) — this describes XLA, not HIGM ^[1]

Flow cytometry analysis for CD18 → LAD; for CD40/CD40L → hyper-IgM ^[6]

Feature	CVID	HIGM
IgM	Usually ↓ (but can rarely be normal/elevated)	↑ or normal
IgG	↓↓↓	↓↓
IgA	↓	↓↓
B cells	Present	Present
Age at onset	Typically later childhood, adolescence, or adulthood	Infancy (after 4–6 months)
Opportunistic infections	Usually no	Yes (Type 1, 3)
Autoimmunity	Yes (haemolytic anaemia, ITP, RA)	Yes
Malignancy	Yes (non-Hodgkin lymphoma)	Yes

Why the confusion arises: CVID is the most common form of severe antibody deficiency ^[1] and shares features of recurrent sinopulmonary and GI infections, autoimmune cytopenias, and lymphoproliferation. However:

CVID is a diagnosis of exclusion — you must rule out HIGM (and XLA, secondary causes) before diagnosing CVID
CVID typically presents later (school age to adulthood), whereas HIGM Type 1 presents in infancy
In CVID, IgM is usually low (not elevated) — if IgM is elevated with low IgG/IgA, think HIGM first
CVID involves impaired B-cell differentiation (cannot form memory B cells or plasma cells effectively), whereas HIGM involves a specific block in CSR

CVID: most common PID with severe antibody deficiency; Ix: ↓↓↓IgG, ↓IgA/E; clinical: highly variable ^[1]

Exam Trap — CVID vs HIGM

A common exam error is to label a child with ↑IgM and ↓IgG as "CVID." If IgM is elevated or normal with profoundly low IgG/IgA/IgE, always think HIGM first. CVID is a diagnosis of exclusion — you must specifically test for CD40L expression and genetic causes of HIGM before settling on CVID.

Feature	SCID	HIGM
Lymphocyte count	↓↓↓ ALC (< 2,500/µL in infants)	Usually normal
T cells	Absent or very low	Normal (in B-cell–intrinsic HIGM types) or normal numbers but dysfunctional signalling (Type 1)
B cells	Variable (depends on subtype)	Present
Thymus	Absent shadow on CXR	Present
Immunoglobulins	All isotypes low	↑IgM, ↓IgG/IgA/IgE
Severity	Fatal without treatment	Variable; Type 1 is severe but less rapidly fatal than SCID

Why the confusion arises: Both SCID and HIGM Type 1 can present with PJP and opportunistic infections in infancy. However:

SCID patients are profoundly lymphopenic (low ALC) with absent T cells and absent thymus on CXR
HIGM Type 1 patients have normal lymphocyte counts and normal T-cell numbers — it is the function of CD40L that is defective, not the T cells themselves
SCID patients usually present earlier (first weeks to months of life) and with a broader range of severe infections (viral, fungal, bacterial, mycobacterial — everything)

SCID: heterogeneous group of disorders with impaired B and T cell development; Ix: ↓ALC, absent thymus shadow on CXR; fatal without treatment ^[1]^[2]

Feature	WAS	HIGM
Inheritance	XLR (WAS gene)	XLR (Type 1) or AR
Platelets	Micro-thrombocytopenia (low AND small)	Normal (though neutropenia in ~50% of Type 1)
IgM	Low	Elevated or normal
IgE	Elevated	Low
Eczema	Yes (hallmark)	No
Classic triad/tetrad	Eczema + micro-thrombocytopenia + immunodeficiency (+ bloody diarrhoea)	Recurrent infections + ↑IgM + ↓IgG/IgA/IgE

Why the confusion arises: WAS is an X-linked PID in boys with recurrent infections. However, WAS has a distinctive tetrad of eczema, micro-thrombocytopenia, immunodeficiency, and bloody diarrhoea ^[2]. The immunoglobulin pattern is different: WAS has low IgM (not high), and elevated IgE. WAS also involves T-cell dysfunction (progressive T-cell lymphopenia) and is classified as a CID with syndromic features rather than a primarily antibody deficiency.

Wiskott-Aldrich syndrome: Eczema, micro-thrombocytopenia → low and small, immunodeficiency, bloody diarrhoea (secondary to thrombocytopenia) ^[2]

Feature	HIES (STAT3 deficiency)	HIGM
IgE	Markedly elevated	Low
IgM	Normal	Elevated or normal
Eosinophils	High absolute eosinophil count	Normal
Infections	Massive cold Staph abscesses, pneumatoceles	Sinopulmonary, PJP, Cryptosporidium
Inheritance	AD (STAT3 loss of function)	XLR or AR
Facial features	Coarse facies, retained primary teeth	No distinctive facies

Why the confusion arises: The name "Hyper-IgE" vs "Hyper-IgM" can be confused on exams. Remember:

HIES: Th17 deficiency → Staphylococcus "cold" abscesses (no warmth/erythema because of poor inflammatory response), CXR showing massive pneumatoceles ^[2]
HIGM: Class-switch defect → ↑IgM, ↓IgG/IgA/IgE

Hyper IgE syndromes: T-cells (Th17) affected; High IgE, high absolute eosinophil count; STAT3 (AD, loss of function); bacteria (massive cold Staph abscess), fungi ^[2]

Feature	DiGeorge	HIGM
Genetics	22q11.2 microdeletion	CD40LG, AICDA, etc.
T cells	Low (thymic hypoplasia/aplasia)	Normal numbers
Associated features	Cardiac abnormalities + abnormal facies + thymic aplasia + cleft palate + hypocalcaemia (CATCH-22)	No cardiac or facial features
Immunoglobulins	Variable (may have low IgG if T-cell help is severely impaired)	↑IgM, ↓IgG/IgA/IgE (specific pattern)

Why the confusion arises: Partial DiGeorge syndrome can cause some degree of humoral immunodeficiency due to impaired T-cell help. However, the presence of conotruncal cardiac anomalies, hypocalcaemia (from parathyroid hypoplasia), facial dysmorphism, and cleft palate distinguishes it readily from HIGM ^[7].

Feature	CGD	HIGM
Defect	Phagocyte NADPH oxidase (cannot produce ROS)	Class-switch recombination
IgG	Normal or elevated	Low
Lymph nodes	↑lymphoid (adenopathy, hepatosplenomegaly)	Variable
Key infections	Staphylococcus, Aspergillus, Burkholderia cepacia, catalase-positive organisms	Encapsulated bacteria, PJP, Cryptosporidium
Diagnostic test	NBT / DHR test	CD40L flow cytometry
BCG	Disseminated BCG (in Hong Kong)	Disseminated BCG (Type 1/3/NEMO)

Why the confusion arises: Both CGD and HIGM can present in infancy with recurrent severe bacterial infections, hepatosplenomegaly, and complications from BCG vaccination in Hong Kong. The AOS past paper highlights CGD presenting with recurrent cervical lymphadenitis, large BCG scar, perianal sepsis, hepatosplenomegaly, and elevated IgG ^[8]. The immunoglobulin pattern is completely different: CGD has elevated IgG (reactive), while HIGM has elevated IgM and low IgG. The NBT/DHR test is diagnostic for CGD ^[6].

CGD = phagocyte don't have NADPH, can't produce ROS, can't kill pathogens ^[2] ↑lymphoid (e.g. adenopathy, hepatosplenomegaly): suggest CGD; ↓lymphoid (e.g. absence of tonsils): suggest XLA, CVID, SCID ^[6]

Feature	Selective IgA deficiency	HIGM
IgA	Low or absent	Low
IgG	Normal	Low
IgM	Normal	Elevated
Clinical	Usually asymptomatic	Symptomatic with recurrent infections
Prevalence	Most common PID	Rare

Why it's in the differential: Selective IgA deficiency is the most common primary immunodeficiency ^[1]. Some patients may have mildly elevated IgM as a compensatory response (since IgA production is impaired and the immune system upregulates IgM mucosal responses). However, IgG is normal in selective IgA deficiency — this is the key distinguishing point.

Always Exclude Secondary Causes Before Diagnosing Primary Immunodeficiency

Before labelling a child with a primary immunodeficiency such as HIGM, you must exclude secondary causes of hypogammaglobulinaemia. This is especially important in the paediatric setting where transient hypogammaglobulinaemia of infancy is common and certain infections or drugs can mimic the immunoglobulin pattern.

Secondary Cause	Mechanism	How to Differentiate from HIGM
HIV infection	HIV destroys CD4+ T cells → impaired T-cell help to B cells → can cause elevated IgM with low IgG in some stages (polyclonal B-cell activation early, then progressive immunodeficiency)	Check HIV status to rule out secondary immunodeficiency ^[6]. HIV serology/PCR; maternal HIV status. In HIV, CD4 count is low and declining; in HIGM Type 1, CD4 count is typically normal
Transient hypogammaglobulinaemia of infancy (THI)	Physiological delay in IgG production after maternal IgG wanes (the "physiological trough" at 3–6 months is exaggerated). IgM is usually normal or slightly elevated	Resolves spontaneously by 2–4 years. IgG is mildly low but not profoundly absent. Vaccine responses eventually normalise. No opportunistic infections
Drug-induced	Phenytoin, carbamazepine, corticosteroids, rituximab → can suppress immunoglobulin production	Drug history is critical. Pattern usually improves after drug withdrawal
Protein-losing states (nephrotic syndrome, protein-losing enteropathy)	Loss of immunoglobulins (all isotypes, particularly IgG which is small enough to be lost) in urine or stool	Nephrotic syndrome: heavy proteinuria, hypoalbuminaemia, oedema. PLE: low albumin, steatorrhoea. IgM is less likely to be elevated (it is a large molecule and less readily lost). Exclude Ig loss in urine, GI, or skin ^[3]
Congenital infections (TORCH — rubella, CMV)	Intrauterine viral infection can damage the immune system → impaired B-cell function; may cause elevated IgM (IgM is the first antibody produced by the fetus, so elevated cord IgM suggests intrauterine infection)	TORCH screen; clinical features (microcephaly, intracranial calcifications, hepatosplenomegaly, petechiae). The elevated IgM is specific anti-pathogen IgM, not a class-switching defect
EBV-associated lymphoproliferative disease	EBV can cause polyclonal B-cell activation → can transiently elevate IgM; can also cause immunoglobulin abnormalities in XLP (X-linked lymphoproliferative disease)	EBV serology/PCR; atypical lymphocytosis on blood film; lymphadenopathy pattern
Malignancy (leukaemia, lymphoma)	Bone marrow infiltration can suppress normal immunoglobulin production; some lymphomas produce monoclonal IgM (e.g., Waldenström macroglobulinaemia — though this is virtually unheard of in paediatrics)	CBC with differential (blasts on PBS), LDH, uric acid, imaging. In children, consider ALL (most common childhood malignancy) if pancytopenia with lymphadenopathy and bone pain ^[8]

This is a key exam skill: given a clinical vignette, which HIGM subtype is it?

Clinical Clue	Think…	Why
Male infant with PJP as first presentation	Type 1 (CD40L deficiency)	X-linked; PJP occurs because macrophage activation is defective (CD40L–CD40 needed for T→macrophage signalling)
Sclerosing cholangitis in a child with HIGM	Type 1 or Type 3	Cryptosporidium requires macrophage killing → defective in CD40L and CD40 deficiency
Neutropenia with oral ulcers	Type 1	~50% of CD40L-deficient patients have cyclical/chronic neutropenia
Massive lymphadenopathy and tonsillar hypertrophy	Type 2 (AID deficiency)	Germinal centres form (CD40L–CD40 intact) but B cells cannot complete CSR → accumulate in germinal centres → lymphoid hyperplasia
Ectodermal dysplasia (sparse hair, conical teeth, anhidrosis)	NEMO deficiency	NEMO (IKKγ) is in the NF-κB pathway, which is also critical for ectodermal development
Female child with HIGM phenotype	Type 2, 3, 4, or 5 (all AR)	If it's a girl, it cannot be X-linked (Type 1 or NEMO) — must be autosomal recessive
Absent opportunistic infections, only sinopulmonary	Type 2, 4, or 5 (B-cell intrinsic)	T-cell–macrophage communication is intact → no susceptibility to PJP or Cryptosporidium
Consanguineous parents	AR forms (Type 2, 3, 4, 5)	Consanguinity ↑ risk of homozygous recessive mutations

Condition	IgM	IgG	B cells	T cells	Key Distinguishing Feature
HIGM (all types)	↑/N	↓↓	Present	Normal numbers	Specific CSR defect; CD40L/CD40/AID testing
XLA	↓↓	↓↓↓	Absent	Normal	No B cells; absent tonsils
CVID	↓	↓↓↓	Present	Normal	Later onset; diagnosis of exclusion
SCID	↓	↓↓	Variable	Absent/↓↓	Profound lymphopenia; absent thymus
WAS	↓	Variable	Present	Progressive ↓	Micro-thrombocytopenia + eczema
HIES	N	N	Present	Normal (Th17 ↓)	Markedly elevated IgE; cold abscesses
DiGeorge	Variable	Variable	Present	↓	CATCH-22 features; cardiac anomalies
CGD	N/↑	N/↑	Present	Normal	Abnormal NBT/DHR; catalase+ infections
Selective IgA def.	N	Normal	Present	Normal	Usually asymptomatic; IgG normal
HIV	Variable	↓ (late)	Present	↓ CD4	HIV serology; declining CD4 count
THI	N/slight ↑	Mild ↓	Present	Normal	Self-resolves by 2–4 years

Approach to diagnosis of PID: Clinical history → Physical examination → Phenotypic investigations (IgG, IgA, IgM levels; vaccine responses; T, B, NK cell enumerations; lymphocyte subsets; cell function e.g. expression of CD40-ligand on activated T cells for hyper-IgM syndrome) → Genetic testing ^[3]

Step-by-step clinical reasoning in a child with suspected HIGM:

History: Age at onset, pattern of infections (sinopulmonary → humoral; opportunistic → combined), family history (X-linked pattern?), failure to thrive, chronic diarrhoea
Examination: Lymphoid tissue assessment (↓lymphoid: suggest XLA, CVID, SCID; ↑lymphoid: suggest CGD ^[6]), oral ulcers/gingivitis (neutropenia), hepatosplenomegaly, dysmorphism (exclude syndromic PID), ENT exam: ear discharge, oral candidiasis; skin: poor wound healing; joints: arthritis ^[6]
First-line bloods: CBC/D (neutropenia?), immunoglobulin profile (the ↑IgM, ↓IgG/IgA/IgE pattern), HIV status ^[6]
Second-line: Lymphocyte subsets (CD3, CD4, CD8, CD19, CD16/56) ^[6], CD40L expression on activated T cells ^[3]^[6], CD40 expression on B cells, vaccine responses
Definitive: Genetic sequencing — identifies the specific gene mutation and enables family studies, prenatal diagnosis ^[3]

High Yield Exam Point — The Immunoglobulin Pattern is Your Anchor

In any PID question, the immunoglobulin pattern is the first and most important clue:

All isotypes low + no B cells → XLA
All isotypes low + no T cells → SCID
↑IgM + ↓IgG/IgA/IgE + B cells present → HIGM
↓IgG with variable IgA/IgM + B cells present + older onset → CVID
Isolated ↓IgA → Selective IgA deficiency
↑IgE + eosinophilia → Hyper-IgE syndrome Then use additional tests (CD40L, AID, genetic) to subtype.

Active Recall - Differential Diagnosis of Hyper-IgM Syndromes

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 410 — Primary Immunodeficiency, XLA, CVID, Hyper-IgM, SCID, Selective IgA deficiency) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (p. 1–2 — HIGM Type 1, WAS, HIES, CGD, SCID) ^[3] Lecture slides: Investigations of Imm Disorders 2025.pdf (p. 10, 21 — Approach to Diagnosis of PID, CD40-ligand expression, genetic testing, exclude secondary causes of Ig loss) ^[6] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 408 — Physical examination and investigation approach to PID, flow cytometry for CD40/CD40L, NBT/DHR, lymphoid assessment) ^[7] Senior notes: Ryan Ho Cardiology.pdf (p. 185 — DiGeorge syndrome, CATCH-22 features, conotruncal cardiac anomalies) ^[8] AOS material: AOS - Paeds.pdf (p. 1 — AOS Q9 CGD past paper question; AOS Q10 ALL presentation)

Diagnostic Criteria, Algorithm, and Investigations for Hyper-IgM Syndromes

1. Diagnostic Criteria

Unlike some conditions (e.g., CVID, which has formal ESID criteria), there are no universally standardised "diagnostic criteria" for HIGM syndromes per se. Diagnosis is established through a combination of:

Characteristic immunoglobulin pattern
Functional protein assays (phenotypic testing)
Confirmatory genetic testing

However, we can construct a practical diagnostic framework based on the ESID/IUIS classification system and the HKU approach taught in lectures.

A diagnosis of Hyper-IgM syndrome requires ALL of the following:

Criterion	Detail	Rationale
A. Clinical features	Recurrent infections (sinopulmonary ± opportunistic) presenting in infancy/early childhood, typically after 4–6 months of age	Maternal IgG protection wanes; the child cannot produce their own IgG/IgA due to class-switch failure
B. Immunoglobulin pattern	High IgM (or normal IgM) with markedly decreased IgG and IgA (± IgE), measured on at least 2 occasions, compared to age-appropriate reference ranges	Single measurement may be confounded by transient infection. Must use paediatric age-specific normals (immunoglobulin levels change dramatically in the first years of life)
C. B cells present	CD19+ B cells are present (> 1%) on flow cytometry	Excludes XLA (where B cells are absent). In HIGM, the B-cell compartment develops normally — the defect is in class switching, not maturation
D. Exclusion of secondary causes	HIV negative, no protein-losing state, no causative drug, no congenital infection	HIV status to r/o 2° immunodeficiency ^[6]. Secondary causes can mimic the ↑IgM, ↓IgG pattern
E. Confirmatory testing	Expression of CD40-ligand on activated T cells (for Type 1) ^[3] OR identification of pathogenic mutation by genetic sequencing	Phenotypic and/or genotypic confirmation identifies the specific subtype and enables family counselling

Phenotypic investigations: IgG, IgA, IgM levels; vaccine responses; T, B, NK cell enumerations; cell function: e.g. expression of CD40-ligand on activated T cells for hyper-IgM syndrome ^[3]

High Yield — GC Lecture Slide Points on IgGAME Interpretation

From GC 144: A child with recurrent infections — Primary immunodeficiencies ^[9]:

High IgM → hyper-IgM
Low IgGAM → agammaglobulinaemia, e.g., XLA
High IgE → hyper-IgE, primary atopic disorder
High IgGAM → chronic granulomatous disease
Low IgM → DOCK8, Wiskott-Aldrich syndrome, ataxia-telangiectasia

This IgGAME pattern interpretation is a core exam framework. The moment you see High IgM with Low IgG/IgA, your differential narrows to HIGM.

For CVID, the ESID has established formal diagnostic criteria (as referenced in the lecture slides ^[3]):

ESID CVID diagnostic criteria require: at least one clinical feature (increased susceptibility to infection, autoimmune manifestations, granulomatous disease, unexplained polyclonal lymphoproliferation, or affected family member) AND marked decrease of IgG and IgA with or without low IgM AND poor antibody response to vaccines or low switched memory B cells AND exclusion of secondary causes AND diagnosis established after the 4th year of life AND no evidence of profound T-cell deficiency ^[3]

HIGM is different because:

It typically presents much earlier (infancy, not after age 4)
The immunoglobulin pattern is distinctive (↑IgM, not pan-hypogammaglobulinaemia)
There are specific gene defects that can be directly tested, making genetic confirmation the gold standard rather than a clinical criteria checklist
CVID is a diagnosis of exclusion; HIGM is a genetically defined entity

The algorithm below integrates the clinical approach taught at HKU, combining the GC 144 lecture slide framework ^[9] with the Investigations of Immunological Disorders 2025 approach ^[3] and the senior notes investigation pathway ^[6].

Genetic testing has an important role in: Diagnosis (All cases), Family studies (HIGM, HAE cases), Prognostication, Prenatal diagnosis, Defining new diseases ^[3]

3. Investigation Modalities — Detailed Breakdown

3.1 STEP 1: First-Line Screening Investigations

These are the investigations you order on any child presenting with a pattern suggestive of primary immunodeficiency. Think of them as your "casting a wide net" tests.

Parameter	Expected Finding in HIGM	Interpretation / Why
Absolute lymphocyte count (ALC)	Usually normal (unlike SCID where ALC is profoundly low)	In HIGM, T and B cells develop and mature normally — the defect is in class switching, not lymphocyte development. ALC low < 3000 (< 1 year) or < 1000 (> 1 year) → severe combined immunodeficiencies ^[9] — this is NOT the pattern in HIGM
Absolute neutrophil count (ANC)	Low in ~50% of HIGM Type 1 (cyclical or chronic neutropenia)	Mechanism: (1) anti-neutrophil autoantibodies (IgM), (2) impaired granulopoiesis, (3) possible marrow effect. ANC < 1 → severe congenital neutropenia ^[9] — note that HIGM neutropenia is usually intermittent rather than as severe as SCN
Platelets	Usually normal	PLT low → Wiskott-Aldrich syndrome ^[9] — if platelets are low AND small (micro-thrombocytopenia), think WAS not HIGM
Absolute eosinophil count (AEC)	Usually normal	AEC high → primary atopic disorders ^[9] or hyper-IgE syndrome
Haemoglobin	May be low if autoimmune haemolytic anaemia complicates	Autoimmune cytopenias are a known complication of HIGM

CBC findings from GC 144 lecture slide ^[9]:

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

AMC low → GATA2 deficiency

PLT low → Wiskott-Aldrich syndrome

Paediatric Normal Values — Age Matters!

In paediatrics, normal lymphocyte counts are much higher than in adults. A "normal-looking" adult lymphocyte count (e.g., 2,000/µL) would actually represent lymphopenia in an infant. Always use age-matched reference ranges:

Infants (< 1 year): normal ALC is typically 4,000–10,000/µL; lymphopenia < 2,500–3,000/µL
Children > 5 years: lymphopenia < 1,500/µL

This is the single most important screening test for HIGM.

Immunoglobulin	Expected Finding	Interpretation
IgM	Elevated or normal for age	B cells produce IgM normally; because they cannot class-switch, IgM may accumulate (especially with chronic antigenic stimulation). However, IgM is not always elevated — "normal" IgM with low other isotypes still raises suspicion
IgG	Markedly decreased (often < 200 mg/dL)	The hallmark. No class switching → no IgG production. Antibody deficiency defined as IgG < 200 mg/dL AND total Ig < 400 mg/dL (OR) complete absence of IgM and IgA ^[6] — in HIGM, IgG is profoundly low while IgM is preserved
IgA	Markedly decreased or absent	IgA requires class switching from IgM. Absent IgA → loss of mucosal immunity → recurrent GI and respiratory infections
IgE	Low or absent	IgE also requires class switching. This distinguishes HIGM from hyper-IgE syndrome (where IgE is markedly elevated)

IgGAME pattern interpretation from GC 144 ^[9]:

High IgM → hyper-IgM

Low IgGAM → agammaglobulinaemia, e.g., XLA

High IgE → hyper-IgE, primary atopic disorder

High IgGAM → chronic granulomatous disease

Must Know — Age-Appropriate Immunoglobulin Reference Ranges

A critical exam trap: you MUST compare immunoglobulin levels to age-matched normal ranges. An IgG of 300 mg/dL might be normal in a 3-month-old (where maternal IgG is still present) but severely low in a 2-year-old. Similarly, IgA is physiologically very low in infancy and reaches adult levels only by 10–16 years. Always check the lab report's age-specific reference range.

Approximate age-specific IgG normals:

Newborn: 700–1,300 mg/dL (maternal transfer)
3–6 months: 200–700 mg/dL (physiological trough)
1 year: 300–1,000 mg/dL
5 years: 500–1,300 mg/dL
Adult: 700–1,600 mg/dL

Investigation	Purpose in HIGM Workup	Expected Finding
HIV serology/PCR	Rule out secondary immunodeficiency ^[6]	Must be negative. In infants < 18 months, use HIV DNA/RNA PCR (not antibody serology, because maternal anti-HIV IgG can cause false positives)
ESR/CRP	Assess for active infection or chronic inflammation ^[6]	May be elevated during active infection; CRP rises with bacterial infection
LFT including GGT	Screen for Cryptosporidium-associated cholangiopathy (HIGM Type 1/3) ^[6]	Elevated ALP/GGT suggests cholangitis; elevated bilirubin suggests cholestasis
Renal function tests	Baseline assessment; some PID patients develop renal complications	Usually normal in HIGM
Globulin level (part of LFT)	Low globulin may suggest hypogammaglobulinaemia ^[6]	Globulin includes immunoglobulins — profoundly low globulin in a child with normal albumin points to low immunoglobulins
CXR	Assess thymus size, lung infections, bronchiectasis ^[6]	Thymus is present in HIGM (unlike SCID where it is absent). Look for bilateral GGO (PJP), consolidation (bacterial pneumonia), or bronchiectasis

This step is critical before proceeding to specific HIGM subtyping. The following must be excluded:

Secondary Cause	Test	Key Point
HIV	HIV antibody (> 18 months) or HIV DNA PCR (< 18 months)	In infants, maternal anti-HIV IgG crosses the placenta → antibody testing unreliable until > 18 months
Drugs	Thorough drug history	Phenytoin, carbamazepine, corticosteroids, rituximab can suppress immunoglobulin production
Protein-losing states	Urine protein/creatinine ratio; stool alpha-1 antitrypsin (protein-losing enteropathy)	Nephrotic syndrome preferentially loses IgG (smaller molecule) while retaining IgM (large pentameric molecule) → can mimic HIGM pattern
Congenital infections	TORCH serology; CMV/rubella PCR	Elevated specific IgM (anti-CMV, anti-rubella) ≠ HIGM. In HIGM, the IgM is polyclonal non-specific

Lymphocyte subset from GC 144 ^[9]:

Low T cells (with or without low B and/or NK) → combined immunodeficiencies

Low B cells only → agammaglobulinaemia

In some forms of CID, T cell 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

Marker	Cell Type	Expected Finding in HIGM	Interpretation
CD3	Pan-T cell	Normal	T cells develop normally in HIGM
CD4	T-helper cells	Normal	CD4+ T cells are present in normal numbers; in Type 1, they are functionally impaired (cannot express CD40L) but numerically intact
CD8	Cytotoxic T cells	Normal
CD19 / CD20	B cells	Normal (> 1%)	B cells are present — this is the key distinction from XLA (where B cells are absent). Low B cells only → agammaglobulinaemia ^[9]
CD16 / CD56	NK cells	Normal	NK cell development is unaffected in HIGM

Lymphocyte subset markers: T-cell: CD3, CD4, CD8; B-cell: CD19, CD20; NK-cell: CD16, CD56 ^[6]

3.4 STEP 4: Specific Functional / Phenotypic Tests (The Diagnostic Key)

This is where you make the definitive phenotypic diagnosis.

This is the most important single confirmatory test for HIGM Type 1 and is specifically highlighted in multiple HKU lecture slides.

Method:

Patient's peripheral blood mononuclear cells (PBMCs) are isolated
T cells are activated in vitro with PMA (phorbol myristate acetate) + ionomycin (or anti-CD3 antibody)
After activation, surface expression of CD40L (CD154) is measured by flow cytometry using a fluorescent anti-CD154 antibody
In normal individuals, activated T cells brightly express CD40L
In HIGM Type 1, CD40L expression is absent or markedly reduced ^[3]^[6]

Why PMA/ionomycin? PMA directly activates protein kinase C, and ionomycin raises intracellular calcium — together they bypass the T-cell receptor and directly activate the intracellular signalling cascade, forcing maximal CD40L surface expression. If CD40L still doesn't appear despite this strong stimulus, the gene product itself must be defective.

Cell function: e.g. expression of CD40-ligand on activated T cells for hyper-IgM syndrome ^[3] Flow cytometry analysis for CD40/CD40L → hyper-IgM ^[6]

High Yield — CD40L Flow Cytometry is the Key Phenotypic Test for HIGM Type 1

In the HKU exam setting, if asked "What is the key investigation to confirm Hyper-IgM Type 1?", the answer is: Flow cytometry for CD40L (CD154) expression on PMA/ionomycin-activated T cells This is directly from both the GC 144 and Investigations of Immunological Disorders 2025 lecture slides.

Test	Method	Finding in HIGM Type 3
CD40 surface expression	Flow cytometry on B cells (CD19+) using anti-CD40 antibody	Absent or reduced → confirms HIGM Type 3 (CD40 deficiency)

This test is simpler than the CD40L test because CD40 is constitutively expressed on B cells (no activation required).

For types 2 (AID) and 5 (UNG), where CD40L and CD40 are both normal, you need to test the intrinsic B-cell CSR machinery:

Patient B cells are stimulated in vitro with CD40L + cytokines (IL-4, IL-10, IL-21)
In normal B cells, this induces CSR and production of IgG/IgA/IgE
In AID-deficient (Type 2) or UNG-deficient (Type 5) B cells, CSR fails despite adequate CD40 stimulation

Vaccine Type	Expected Response in HIGM	Rationale
Protein vaccines (tetanus, diphtheria, hepatitis B)	Absent or very poor IgG response despite adequate vaccination	IgG production requires CSR — this is blocked in HIGM. May have IgM response to vaccination but NO IgG
Polysaccharide vaccines (pneumococcal polysaccharide)	Absent IgG response	T-independent B-cell responses can produce some IgM but not IgG in HIGM
Isohemagglutinins (anti-A, anti-B blood group antibodies)	May be present (these are naturally occurring IgM antibodies)	Isohemagglutinins are IgM class → they can still be produced in HIGM. This distinguishes HIGM from XLA (where isohemagglutinins are absent because no B cells exist)

Humoral immunity assessment: vaccine titre, Coomb's test and isohemagglutinin, IgG subclass or specific Ig level ^[6]

Flow cytometry can quantify switched memory B cells (CD19+CD27+IgD−):

In HIGM, these are profoundly reduced or absent because B cells cannot undergo CSR
This is useful but not specific — switched memory B cells are also reduced in CVID

Genetic testing has an important role in: Diagnosis (All cases), Family studies (HIGM, HAE cases), Prognostication, Prenatal diagnosis, Defining new diseases ^[3]

Gene	HIGM Type	Method	Implications
CD40LG	Type 1	Sanger sequencing or NGS/WES	Identifies specific mutation; enables carrier detection in female relatives (X-linked); prenatal diagnosis
AICDA	Type 2	Sanger sequencing or NGS/WES	AR inheritance → recurrence risk 25% for each subsequent pregnancy
CD40 (TNFRSF5)	Type 3	Sanger sequencing or NGS/WES	AR; clinically similar to Type 1
UNG	Type 5	Sanger sequencing or NGS/WES	AR
IKBKG (NEMO)	NEMO deficiency	Sanger sequencing or NGS/WES	X-linked; associated ectodermal dysplasia

In practice, most centres now use a targeted immunodeficiency gene panel or whole exome sequencing (WES) rather than sequential single-gene testing, as this is faster and can identify novel or unexpected mutations.

Many IEIs require tailored investigations, and cannot be picked up by initial screening tests. Always consult immunology when clinically suspicious ^[9]

Practical Point — Always Consult Immunology

The GC 144 lecture explicitly states: "Many IEIs require tailored investigations, and cannot be picked up by initial screening tests. Always consult immunology when clinically suspicious." ^[9] In clinical practice, once you suspect HIGM based on the immunoglobulin pattern, refer early to a paediatric immunologist for the specialised functional and genetic testing.

Once HIGM is diagnosed (particularly Type 1 or Type 3), systematically screen for complications:

Complication	Investigation	Key Finding	Clinical Significance
Cryptosporidium infection	Stool microscopy (modified Ziehl-Neelsen acid-fast stain); stool PCR for Cryptosporidium	Oocysts in stool; positive PCR	Leading cause of sclerosing cholangitis and death in HIGM Type 1
Sclerosing cholangitis	LFT (ALP, GGT elevated); abdominal ultrasound (biliary dilation); MRCP (beading, strictures of intrahepatic bile ducts)	Elevated GGT/ALP + biliary changes on imaging	Progressive liver disease → may require liver transplant
Bronchiectasis	HRCT chest (gold standard); lung function tests (spirometry if age-appropriate)	Signet-ring sign, tram-track lines, bronchial wall thickening	Irreversible airway damage from recurrent infections
Autoimmune cytopenias	Direct Coombs test (DAT); reticulocyte count; LDH, haptoglobin; platelet count	Positive DAT → AIHA; low platelets → ITP	Common in all HIGM subtypes
Malignancy	Clinical surveillance; imaging as indicated; consider periodic liver imaging if chronic cholangitis	Lymphadenopathy, hepatomegaly, rising LDH	Increased risk of lymphoma and hepatobiliary malignancies
PJP	CXR (bilateral GGO), HRCT if CXR equivocal; induced sputum or BAL for Pneumocystis PCR/silver stain; serum β-D-glucan, LDH	GGO on imaging + positive PCR/microscopy	May be the presenting illness in HIGM Type 1

Investigation	Normal Result → Excludes	Abnormal Result → Suggests	HIGM-Specific Pattern
CBC/D	Normal ALC → excludes SCID; Normal ANC → no neutropenia	Low ANC → neutropenia (HIGM Type 1); Low ALC → SCID; Low platelets → WAS	ALC normal, ANC may be low in Type 1
Ig profile	Normal IgGAME → excludes humoral PID	↑IgM + ↓IgG/IgA/IgE → HIGM; Pan-hypo → XLA or CVID	↑IgM, ↓↓IgG, ↓↓IgA, ↓IgE
B cells (CD19+)	Present → excludes XLA	Absent → XLA	Present in HIGM
T cells (CD3/CD4/CD8)	Normal → excludes SCID, DiGeorge	Low → SCID, DiGeorge	Normal in HIGM
CD40L on activated T cells	Normal → excludes HIGM Type 1	Absent → HIGM Type 1	Key test for Type 1
CD40 on B cells	Normal → excludes HIGM Type 3	Absent → HIGM Type 3
Vaccine IgG responses	Protective titres → excludes significant Ab deficiency	Absent IgG responses → confirms antibody deficiency	IgM responses may be present; IgG responses absent
Genetic sequencing	No pathogenic variant → consider Type 4 or novel gene	Pathogenic variant identified → definitive diagnosis	Gold standard for all subtypes

5. Special Considerations in Paediatric Investigation

Immunoglobulin levels are physiologically low in early infancy (the "physiological trough" at 3–6 months). If you suspect HIGM in a young infant, you may need to repeat testing at an older age or rely more heavily on functional tests (CD40L expression) and genetics
CD40L expression testing can be done at any age, even in neonates, and is not affected by maternal immunoglobulin levels

Current newborn screening programmes (TREC for SCID, KREC for B-cell deficiencies) will NOT detect HIGM:
- TREC is normal (T cells develop normally)
- KREC is normal (B cells develop normally)
Therefore, HIGM requires clinical suspicion — the child must become symptomatic before diagnosis

If a child has already received live vaccines (BCG, MMR, OPV, rotavirus, varicella) before diagnosis, monitor for adverse reactions:
- Disseminated BCG → consider HIGM Type 1/3 or NEMO deficiency
- Vaccine-strain infection → indicates combined immunodeficiency component
Vaccine responses are tested using killed/subunit vaccines (tetanus, diphtheria, pneumococcal) — these are safe in immunodeficient patients

Genetic testing for HIGM has implications beyond the index patient: carrier status for mother and sisters (in X-linked forms), prenatal diagnosis for future pregnancies
In Hong Kong, this requires informed consent from parents/guardians with appropriate pre-test genetic counselling
For adolescents (typically ≥ 12 years), assent should be sought in addition to parental consent

High Yield Summary — Diagnostic Approach to HIGM

The diagnostic algorithm in 6 steps:

Screen: CBC/D (neutropenia?) + Immunoglobulin profile (↑IgM, ↓IgG/IgA/IgE) ^[9]
Exclude secondary causes: HIV, drugs, protein loss, congenital infection ^[6]
Lymphocyte subsets: B cells present (excludes XLA), T cells normal (excludes SCID) ^[6]^[9]
Specific functional test: CD40L expression on activated T cells (Type 1) or CD40 on B cells (Type 3) ^[3]^[6]
Genetic confirmation: Sequencing of CD40LG, AICDA, CD40, UNG, or IKBKG — definitive for all subtypes ^[3]
Complication screening: Cryptosporidium (stool PCR), liver (LFT + USS/MRCP), lungs (HRCT), autoimmune cytopenias (DAT)

Key exam point: "Many IEIs require tailored investigations, and cannot be picked up by initial screening tests. Always consult immunology when clinically suspicious." ^[9]

Active Recall - Diagnosis and Investigations of Hyper-IgM Syndromes

References

^[3] Lecture slides: Investigations of Imm Disorders 2025.pdf (p. 10 — Approach to Diagnosis of PID, CD40L expression, genetic testing; p. 16 — ESID CVID diagnostic criteria) ^[6] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 408 — Physical examination and investigation approach to PID, CBC/D, Ig profile, lymphocyte subsets, flow cytometry for CD40/CD40L, HIV status, CXR, second-line investigations) ^[9] GC lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p. 50 — CBC interpretation for IEI, IgGAME pattern interpretation, lymphocyte subset interpretation, 2024 update)

Management of Hyper-IgM Syndromes

Management of HIGM syndromes rests on five pillars:

Immunoglobulin replacement therapy — replaces the missing IgG (and to a lesser extent IgA)
Antimicrobial prophylaxis — prevents opportunistic infections (especially in Type 1/3 where macrophage activation is defective)
Haematopoietic stem cell transplantation (HSCT) — the only curative treatment
Complication surveillance and management — sclerosing cholangitis, autoimmune cytopenias, bronchiectasis, malignancy
Supportive and family-centred care — genetic counselling, psychosocial support, infection avoidance measures, vaccine guidance

The management approach differs critically by HIGM subtype, because the subtypes with T-cell–macrophage signalling defects (Types 1, 3, NEMO) require more aggressive treatment than the purely B-cell intrinsic subtypes (Types 2, 4, 5).

3. Treatment Modalities

3.1 Immunoglobulin Replacement Therapy

This is the cornerstone of management for all HIGM subtypes. The logic is straightforward: the patient cannot produce IgG → we provide exogenous IgG.

HIGM patients have profoundly low IgG and IgA → susceptible to encapsulated bacteria, Giardia, and other pathogens cleared by antibody-mediated opsonisation
Replacement IgG provides passive humoral immunity (opsonisation, complement activation, toxin neutralisation)
IgG replacement does NOT correct the T-cell/macrophage defect in Types 1/3 → opportunistic infections (PJP, Cryptosporidium) still occur despite adequate IgG levels → hence the need for additional prophylaxis

IVIG / SCIG — listed as treatment for antibody deficiencies including XLA in the lecture notes ^[2]

Route	Regimen	Advantages	Disadvantages
IVIG (Intravenous Immunoglobulin)	400–600 mg/kg every 3–4 weeks	Reliable trough levels; can give large volumes; hospital-supervised	Requires IV access (distressing for young children); infusion reactions (fever, chills, headache, rarely anaphylaxis); need to attend hospital
SCIG (Subcutaneous Immunoglobulin)	100–150 mg/kg weekly (or equivalent divided over more frequent smaller doses)	Home-based (parents trained to administer); more stable IgG levels (no peak-trough fluctuation); fewer systemic side effects; better quality of life for families	Local site reactions (swelling, erythema at injection site); requires patient/parent training; smaller volumes per site (multiple sites may be needed)

Paediatric considerations:

SCIG is increasingly preferred in paediatrics because it avoids repeated IV cannulation and can be administered at home, reducing hospital visits — this is particularly important for family-centred care and minimising disruption to the child's schooling and normal activities
Infants and small children have proportionally less subcutaneous tissue → may need multiple infusion sites
Target trough IgG level: maintain > 500–800 mg/dL (some centres target > 800 mg/dL for patients with chronic lung disease or bronchiectasis). Trough levels are measured just before the next IVIG dose (or at any time for SCIG, since levels are more stable)

IgG replacement is lifelong unless the patient undergoes successful HSCT with full immune reconstitution
It provides IgG only — it does not replace IgA (IgA is present in very small amounts in IVIG/SCIG preparations and does not reach mucosal surfaces effectively). Therefore, mucosal immunity remains impaired
IgA-deficient patients receiving IVIG can rarely develop anti-IgA antibodies → anaphylaxis on subsequent infusions. This is more of a concern in selective IgA deficiency than in HIGM, but worth noting
Monitor: trough IgG levels, clinical infection frequency, LFTs, renal function

Contraindication/Caution	Reason
Known IgA deficiency with anti-IgA antibodies	Risk of anaphylaxis (rare in HIGM context as IgA is low but anti-IgA antibodies may form)
Acute renal impairment	IVIG is hyperosmolar and can cause osmotic nephropathy (especially sucrose-containing preparations)
Fluid overload risk	Large-volume IV infusions; use concentrated preparations in small children and those with cardiac disease
History of thromboembolic events	IVIG increases blood viscosity; risk of VTE (more relevant in adults)

3.2 Antimicrobial Prophylaxis

Prophylaxis is essential in HIGM Types 1, 3, and NEMO deficiency because these patients have a combined immunodeficiency (defective macrophage activation). Prophylaxis is less routinely needed in Types 2, 4, 5 (pure humoral defects), though it may be given if infections are frequent.

Drug	Dose (Paediatric)	Rationale	Key Points
Co-trimoxazole (TMP-SMX / Septrin)	TMP component: 5 mg/kg/day given on 3 consecutive days per week (or daily in some centres)	Prevents Pneumocystis jirovecii pneumonia, which is the most common presenting infection and a major cause of morbidity in HIGM Type 1. PJP occurs because defective CD40L→CD40 signalling impairs macrophage killing of Pneumocystis	Must check G6PD status before starting — SMX is a sulphonamide and can cause haemolysis in G6PD-deficient patients (relevant in Hong Kong where G6PD deficiency is common, ~4–5% of males). Also provides some activity against Toxoplasma and common respiratory bacteria

In Hong Kong, G6PD deficiency is screened at birth via the newborn metabolic screening programme. Always check the result before prescribing co-trimoxazole. If G6PD-deficient, use alternatives (dapsone — but also check G6PD! — or atovaquone or aerosolised pentamidine in older children).

Target Pathogen	Prophylactic Agent	Paediatric Dose	Indication
Fungi (Candida, Aspergillus)	Fluconazole or itraconazole	Fluconazole 3–6 mg/kg/day	Types 1/3 with recurrent oral/systemic candidiasis or if starting immunosuppression
Cryptosporidium	No effective prophylactic drug → water precautions are the mainstay	N/A	Advise families to: boil or filter drinking water, avoid untreated water sources, avoid swimming in potentially contaminated water. Cryptosporidium oocysts are resistant to standard chlorination
Bacterial infections	Consider prophylactic antibiotics (e.g., azithromycin)	Azithromycin 10 mg/kg once weekly or 5 mg/kg daily	If recurrent bacterial infections despite adequate IgG replacement; azithromycin also has immunomodulatory properties beneficial in bronchiectasis

Critical — Cryptosporidium Prevention

There is no proven drug prophylaxis for Cryptosporidium. Prevention relies on:

Boiled or filtered water (oocysts are chlorine-resistant but killed by boiling at 72°C for 1 minute)
Avoiding recreational water (swimming pools, lakes)
Hand hygiene
Early HSCT (definitive cure) before Cryptosporidium cholangitis develops

Once Cryptosporidium sclerosing cholangitis is established, it is often irreversible even with immune reconstitution after HSCT. This is why early HSCT is advocated in HIGM Type 1 — ideally before the child develops chronic cholangiopathy.

~50% of HIGM Type 1 patients develop cyclical or chronic neutropenia
If ANC < 500/µL and associated with oral ulcers, gingivitis, or recurrent bacterial infections:
- G-CSF (granulocyte colony-stimulating factor): 5–10 µg/kg/day subcutaneously, titrated to maintain ANC > 1,000/µL
- Mechanism: G-CSF stimulates neutrophil production from bone marrow precursors and enhances neutrophil survival and function
If neutropenia is autoimmune (anti-neutrophil antibodies), IVIG itself may help (Fc receptor blockade, similar to ITP treatment)

3.3 Haematopoietic Stem Cell Transplantation (HSCT) — The Curative Option

HSCT is the only curative treatment for HIGM syndromes. It replaces the patient's defective immune system with a donor-derived immune system that has functional CD40L (or CD40, AID, etc.), restoring:

Normal class-switch recombination → IgG/IgA/IgE production
Normal T-cell–macrophage interaction → protection against opportunistic infections
Normal neutrophil production (resolves autoimmune neutropenia)

HSCT — listed as treatment for SCID ^[2]; the same curative principle applies to HIGM Type 1/3

Strong Indication	Rationale
HIGM Type 1 (CD40L deficiency)	Combined immunodeficiency with high risk of Cryptosporidium cholangitis, PJP, and progressive organ damage. Best outcomes when transplanted early (before age 2–5 years), before irreversible complications develop
HIGM Type 3 (CD40 deficiency)	Clinically identical to Type 1 — same rationale applies
NEMO deficiency with severe phenotype	Ectodermal dysplasia with significant immunodeficiency

Relative/Less Common Indication	Rationale
HIGM Type 2 (AID deficiency)	Purely humoral defect; most patients do well on IgG replacement alone. HSCT considered only if severe infections despite optimal replacement, or progressive lymphoproliferation
HIGM Type 5 (UNG deficiency)	Similar to Type 2 — HSCT rarely needed

The earlier the better — ideally before:
- Cryptosporidium sclerosing cholangitis develops (once established, liver damage is irreversible and may preclude HSCT)
- Chronic lung disease/bronchiectasis becomes established
- Significant end-organ damage accumulates
Optimal window: before age 2–5 years for best survival and engraftment outcomes
Overall survival after HSCT for HIGM Type 1: approximately 70–80% with matched sibling donors; lower (~50–60%) with matched unrelated donors or haploidentical donors, though outcomes are improving with modern conditioning regimens

Donor Type	Priority	Notes
Matched sibling donor (MSD)	First choice	Best outcomes; lowest GVHD risk. Must confirm sibling is not affected (genetic testing) and is HLA-matched
Matched unrelated donor (MUD)	Second choice	Available via international registries (e.g., NMDP, BMDW). Requires 10/10 HLA match
Haploidentical donor (parent)	Third choice	Increasingly used with improved T-cell depletion techniques (e.g., TCRαβ+/CD19+ depletion); higher GVHD risk
Cord blood	Alternative	Higher rate of graft failure; lower GVHD risk; no need for perfect HLA match

Myeloablative conditioning (e.g., busulfan + cyclophosphamide ± fludarabine) is generally used to ensure full donor engraftment
Reduced-intensity conditioning (RIC) may be used in patients with significant organ damage (e.g., liver disease) who cannot tolerate full myeloablation
The goal is to ablate the recipient's immune system sufficiently for donor cells to engraft and produce a functional immune system

Parameter	What to Monitor	Why
Engraftment	Donor chimerism (percentage of donor-derived cells) by STR analysis	Must achieve full or stable mixed chimerism for immune reconstitution
Immune reconstitution	Serial immunoglobulin levels, lymphocyte subsets, vaccine responses	Confirms the new immune system is producing IgG/IgA/IgE (class switching restored)
GVHD	Skin, liver, GI symptoms; liver function tests	Graft-versus-host disease is a major transplant complication, especially with unrelated/mismatched donors
Infections	Viral reactivation (CMV, EBV, adenovirus), bacterial, fungal	Immunosuppression during conditioning and engraftment period increases infection risk
IgG replacement	Continue IVIG/SCIG until B-cell reconstitution and adequate IgG production confirmed	May take 6–12+ months post-HSCT for humoral immunity to fully recover

Contraindication	Reason
Advanced sclerosing cholangitis / liver failure	Transplant-related mortality is very high; may need liver transplant first or combined liver + HSCT
Active uncontrolled infection	Must achieve infection control before conditioning (conditioning causes further immunosuppression)
Severe irreversible organ damage	If quality of life improvement unlikely, risk–benefit may not favour HSCT

High Yield — Why Early HSCT in HIGM Type 1?

The single most important reason to transplant early is to prevent Cryptosporidium sclerosing cholangitis. Once cholangitis is established:

It is often irreversible (fibrotic biliary damage does not reverse after immune reconstitution)
It progresses to liver failure and/or cholangiocarcinoma
It significantly increases transplant-related mortality Therefore, when HIGM Type 1 is diagnosed, HSCT should be discussed immediately with the family, even if the child appears clinically well.

3.4 Emerging / Investigational Therapies

Gene therapy for HIGM Type 1 (CD40L deficiency) is under active investigation but not yet in clinical use (as of 2025–2026)
The challenge: CD40L expression must be tightly regulated — constitutive overexpression of CD40L causes lymphoproliferation and autoimmunity in animal models
Research is focused on developing regulated lentiviral vectors that express CD40L only when T cells are activated
This remains a promising future therapy but is not currently available outside clinical trials

Letermovir — prophylaxis against CMV reactivation post-HSCT (approved for CMV-seropositive HSCT recipients)
Secukinumab (anti-IL-17A monoclonal antibody) — used in hyper-IgE syndrome (not in HIGM), but mentioned in the lecture notes for HIES ^[2]
No specific biologic therapy is currently approved for HIGM syndromes themselves

Complication	Management	Paediatric Considerations
PJP (acute treatment)	High-dose co-trimoxazole IV: TMP 15–20 mg/kg/day in 3–4 divided doses for 21 days. Add corticosteroids (prednisolone 1–2 mg/kg/day) if hypoxic (PaO2 < 70 mmHg or A-a gradient > 35). Switch to PO when clinically stable. Must check G6PD status	IV co-trimoxazole is diluted in 500 mL infusion — watch for fluid overload in small children (reduce infusion volume or use concentrated preparation)
Cryptosporidium diarrhoea/cholangitis	Nitazoxanide (first-line): 100 mg BD (1–3 years), 200 mg BD (4–11 years), 500 mg BD (≥ 12 years) for 3 days minimum (may need prolonged courses). ± Azithromycin. Definitive: immune reconstitution via HSCT — drugs alone often fail to eradicate Cryptosporidium in immunocompromised patients	Nitazoxanide is available as an oral suspension (paediatric formulation). Hydration support for chronic diarrhoea. Monitor LFTs/GGT for progressive cholangiopathy
Autoimmune haemolytic anaemia / ITP	First-line: Corticosteroids (prednisolone 1–2 mg/kg/day). Second-line: High-dose IVIG (1–2 g/kg — the patient is already on replacement IVIG, but additional high-dose IVIG works via Fc receptor blockade on macrophages). Third-line: Rituximab (anti-CD20, depletes B cells — use with caution as patient is already immunodeficient); mycophenolate, sirolimus	In HIGM, autoimmune cytopenias can be challenging because the patient is already immunodeficient — aggressive immunosuppression risks worsening infections
Neutropenia	G-CSF (filgrastim 5–10 µg/kg/day SC) if ANC < 500 and symptomatic	Titrate to maintain ANC > 1,000. Monitor for splenomegaly with long-term G-CSF
Bronchiectasis	Chest physiotherapy (daily airway clearance); prompt treatment of acute exacerbations with antibiotics (guided by sputum culture); azithromycin as long-term immunomodulatory/anti-inflammatory agent (3 days/week); ensure adequate IgG trough levels (target > 800 mg/dL)	Teach parents and child airway clearance techniques. Spirometry for monitoring in school-age children. Annual HRCT if clinically indicated
Sclerosing cholangitis	Ursodeoxycholic acid (UDCA) 10–20 mg/kg/day — may slow progression (evidence limited). Treat Cryptosporidium if still present. Liver transplant if end-stage liver disease develops	Combined liver transplant + HSCT may be needed in severe cases
Malignancy (lymphoma, HCC)	Standard oncological management; note that chemotherapy carries higher infection risk in an immunodeficient patient	Surveillance for lymphoproliferation at each clinic visit

3.6 Infection Avoidance and Vaccination Guidance

Vaccine	Status	Why
BCG	Contraindicated	Risk of disseminated BCG disease (especially Types 1, 3, NEMO). In Hong Kong, BCG is given at birth — may have already been administered before diagnosis
MMR (measles, mumps, rubella)	Contraindicated	Risk of vaccine-strain infection
OPV (oral polio vaccine)	Contraindicated (use IPV instead)	Risk of vaccine-derived poliovirus. Hong Kong uses IPV (inactivated) in the routine schedule, so this is less of a concern locally
Rotavirus	Contraindicated	Risk of prolonged vaccine-strain shedding/infection
Varicella / Zoster	Contraindicated	Risk of disseminated varicella
LAIV (live attenuated influenza vaccine)	Contraindicated	Use inactivated influenza vaccine instead

Can be given safely but are unlikely to produce protective IgG responses (since class switching is defective)
Still recommended because:
- May induce some IgM-mediated protection
- Post-HSCT, these vaccinations will need to be repeated once the new immune system is functional
Household contacts should be fully vaccinated (including annual influenza vaccine) to provide cocoon protection

Measure	Rationale
Boil drinking water / use filtered water	Prevent Cryptosporidium ingestion (chlorine-resistant)
Hand hygiene	Standard infection prevention
Avoid contact with sick individuals	Minimise exposure to respiratory pathogens
Avoid swimming in natural water / poorly maintained pools	Cryptosporidium risk
Prompt medical attention for fevers	Febrile illness in an immunodeficient child must be taken seriously — empiric antibiotics may be needed
School/nursery notification	Alert teachers to child's immunocompromised status; plan for outbreaks (e.g., varicella, measles — child should be excluded and may need post-exposure prophylaxis with VZIG or immunoglobulin)

3.7 Supportive and Family-Centred Care

Inheritance	Counselling Points
X-linked (Type 1, NEMO)	Mother is an obligate carrier (if the mutation is inherited, not de novo). Each son has a 50% chance of being affected; each daughter has a 50% chance of being a carrier. Offer carrier testing to maternal aunts and female siblings when they reach appropriate age
Autosomal recessive (Type 2, 3, 4, 5)	Both parents are carriers. Each subsequent child has a 25% chance of being affected. Consanguinity increases risk
Prenatal diagnosis	Available via chorionic villus sampling (CVS) at 10–12 weeks or amniocentesis at 15–18 weeks, with targeted genetic testing for the known family mutation

Genetic testing has an important role in: Diagnosis (All cases), Family studies (HIGM, HAE cases), Prognostication, Prenatal diagnosis ^[3]

Chronic disease impact: HIGM is a lifelong condition requiring regular hospital visits, infusions, and medication → impacts the child's quality of life, schooling, and family dynamics
Mental health: Screen for anxiety and depression in both the child (adolescents) and parents/caregivers
Peer support: Connect families with PID patient organisations (e.g., Hong Kong PID patient support group, Jeffrey Modell Foundation)
Transition planning: As the child approaches adolescence, begin planning transition from paediatric to adult immunology services

Growth charts: Plot at every visit. Chronic infection, malabsorption (Cryptosporidium-related), and chronic disease can cause failure to thrive (FTT)
Development: Most HIGM children have normal neurodevelopment (unlike some syndromic PIDs like DiGeorge or ataxia-telangiectasia). However, chronic illness, frequent hospitalisations, and school absences can impact learning → advocate for educational support

Modality	Type 1 (CD40L)	Type 2 (AID)	Type 3 (CD40)	Type 5 (UNG)	NEMO
IgG replacement	Lifelong (or until HSCT)	Lifelong	Lifelong (or until HSCT)	Lifelong	Lifelong (or until HSCT)
PJP prophylaxis	Yes — co-trimoxazole	Usually not needed	Yes	Usually not needed	Yes
Antifungal prophylaxis	Consider	Usually not needed	Consider	Usually not needed	Consider
Water precautions	Essential	Not typically needed	Essential	Not typically needed	Essential
G-CSF	If neutropenic	Rarely needed	If neutropenic	Rarely needed	If neutropenic
HSCT	Strongly recommended (early)	Rarely needed	Strongly recommended	Rarely needed	Case-by-case
Live vaccines	Contraindicated	Contraindicated	Contraindicated	Contraindicated	Contraindicated
Prognosis without HSCT	Poor (~20% survive to age 25)	Better (predominantly humoral)	Poor (similar to Type 1)	Better	Variable
Prognosis with HSCT	~70–80% survival (MSD)	N/A (rarely transplanted)	~70–80% (MSD)	N/A	Variable

High Yield Summary — Management of HIGM

All subtypes: Lifelong IgG replacement (IVIG or SCIG) — target trough IgG > 500–800 mg/dL
Types 1, 3, NEMO: Additional co-trimoxazole PJP prophylaxis (check G6PD first!), water precautions for Cryptosporidium, antifungal prophylaxis if recurrent candidiasis, G-CSF if neutropenic
Curative: HSCT — strongly recommended for Types 1, 3, NEMO. Best outcomes when done early (before age 2–5 years), before irreversible Cryptosporidium cholangitis develops
Live vaccines are CONTRAINDICATED in all subtypes; household contacts should be fully vaccinated (cocoon protection)
No effective drug prophylaxis for Cryptosporidium — water safety is the key prevention strategy
Genetic counselling is essential: carrier detection in female relatives (X-linked forms), prenatal diagnosis for future pregnancies
Complication surveillance: LFTs + GGT (cholangiopathy), stool Cryptosporidium PCR, HRCT (bronchiectasis), DAT (autoimmune cytopenias), growth monitoring

Active Recall - Management of Hyper-IgM Syndromes

References

^[2] Senior notes: Jerry's immunodeficiencies.pdf (p. 2 — Treatment column: HSCT for SCID, IVIG/SCIG for antibody deficiencies, Septrin + IVIG + secukinumab for HIES) ^[3] Lecture slides: Investigations of Imm Disorders 2025.pdf (p. 10 — Genetic testing role: diagnosis, family studies for HIGM, prognostication, prenatal diagnosis) ^[9] GC lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf (p. 50 — CBC interpretation, IgGAME interpretation, "Many IEIs require tailored investigations, always consult immunology when clinically suspicious")

Complications of Hyper-IgM Syndromes

Complications in HIGM are the principal determinants of morbidity and mortality. Understanding them from first principles requires remembering the two axes of the disease:

Humoral deficiency (absent IgG/IgA → recurrent bacterial infections → chronic organ damage)
Defective T-cell/macrophage activation (Types 1, 3, NEMO → opportunistic infections → unique complications not seen in purely humoral PIDs)

The complications can be organised into infectious, hepatobiliary, pulmonary, autoimmune/immune dysregulation, malignant, and treatment-related categories.

1. Infectious Complications

These are the most immediate and frequent complications, beginning in infancy.

Feature	Detail
Subtypes affected	Types 1, 3, NEMO (those with defective macrophage activation)
Mechanism	CD40L–CD40 signalling is required for macrophage activation against intracellular pathogens. Without this signal, alveolar macrophages cannot clear Pneumocystis organisms from the lung. Pneumocystis is a fungus (genus fungi, resistant to standard antifungals) that colonises the alveolar epithelium → progressive alveolar filling → impaired gas exchange
Clinical presentation	Subacute onset of progressive dyspnoea, tachypnoea, dry non-productive cough, fever, hypoxia. In infants: tachypnoea, feeding difficulty, intercostal recession, desaturation
Imaging	CXR: bilateral perihilar or diffuse ground-glass opacities (GGO). HRCT: bilateral GGO with peripheral sparing
Diagnosis	Cannot be cultured. Induced sputum or BAL for silver stain / immunofluorescence / PCR. Serum β-D-glucan (elevated), LDH (elevated)
Significance	Often the presenting illness of HIGM Type 1 — the first episode of PJP in an infant should trigger immunological workup. PJP carries significant mortality if untreated

PJP is the classic presenting infection in HIGM Type 1 — remember that immunodeficiency is not just infection-susceptible, also increased risk for autoimmune diseases, malignancy etc. ^[2]

Feature	Detail
Subtypes affected	Types 1, 3, NEMO
Mechanism	Cryptosporidium parvum is an intracellular protozoan parasite that colonises epithelial cells of the GI tract and biliary tree. Clearance requires CD40L–CD40-mediated macrophage and dendritic cell activation. Without this, the parasite establishes chronic persistent infection → progressive tissue damage
GI manifestation	Chronic watery diarrhoea → malabsorption → failure to thrive. Dehydration can be severe in young children
Hepatobiliary manifestation	Sclerosing cholangitis — see Section 2 below
Diagnosis	Modified Ziehl-Neelsen acid-fast stain of stool (oocysts appear as red round structures); stool PCR (more sensitive)
Prevention	No effective drug prophylaxis exists. Prevention relies on water safety: boil/filter drinking water (oocysts resist standard chlorination), avoid recreational water, hand hygiene

Feature	Detail
Subtypes affected	All types (due to IgG/IgA deficiency)
Mechanism	IgG deficiency → loss of opsonisation → encapsulated bacteria cannot be phagocytosed efficiently. IgA deficiency → loss of mucosal barrier at respiratory and GI surfaces
Common organisms	Encapsulated bacteria: Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis ^[1]. Also Giardia lamblia (cleared by IgA)
Common sites	Sinopulmonary: otitis media, sinusitis, pneumonia. GI: chronic diarrhoea (Giardia)
Long-term consequence	Recurrent infections → chronic airway inflammation → bronchiectasis (see Section 3)

Commonly cause sinopulmonary and GI infections — sites where most Ig deposits, open to external environment ^[1]

Pathogen	Clinical Manifestation	Mechanism
Candida spp.	Oral thrush, oesophageal candidiasis, systemic candidiasis	Defective Th1/macrophage responses
CMV	Disseminated CMV infection, hepatitis, colitis, retinitis	Impaired CD8+ T-cell priming (CD40L–CD40 on dendritic cells is needed for cross-presentation)
Mycobacteria (including BCG)	Disseminated BCG disease (in Hong Kong where BCG is given at birth); local or disseminated mycobacterial infection	Macrophage activation defect → cannot form effective granulomas → mycobacteria persist and disseminate
Enterovirus	Chronic meningoencephalitis	IgG deficiency → cannot neutralise enterovirus in the CNS
Toxoplasma gondii	Cerebral toxoplasmosis (rare in children)	Requires cell-mediated immunity for control

Hong Kong Relevance — Disseminated BCG

In Hong Kong, BCG is administered at birth as part of the routine childhood immunisation schedule. An infant with undiagnosed HIGM Type 1 may present with disseminated BCG disease — regional lymphadenitis progressing to suppurative lymphadenitis, osteitis, or systemic BCG-osis. This can be the first clinical clue to the diagnosis, and should prompt immediate immunological investigation.

2. Hepatobiliary Complications

This is the most feared and most important category of complications in HIGM Type 1, as it is the leading cause of death.

Feature	Detail
Subtypes affected	Types 1, 3 (those with defective macrophage activation allowing chronic Cryptosporidium persistence)
Pathogenesis	Cryptosporidium colonises the biliary epithelium → chronic inflammation of the bile ducts → progressive periductal fibrosis → strictures → obstructive cholangiopathy. The process is analogous to primary sclerosing cholangitis (PSC) in adults but is infection-driven rather than autoimmune
Why it is irreversible	Once fibrosis and strictures are established, the architectural damage to the bile ducts does not reverse — even if Cryptosporidium is subsequently cleared (e.g., after HSCT and immune reconstitution). The fibrotic process becomes self-perpetuating through ongoing inflammation, bile stasis, and secondary bacterial cholangitis
Clinical features	Progressive cholestatic jaundice (elevated conjugated bilirubin), pruritus, hepatomegaly, right upper quadrant pain. Late: portal hypertension (ascites, splenomegaly, variceal bleeding), hepatic failure
Investigations	LFTs: elevated GGT and ALP (cholestatic pattern), elevated bilirubin. USS: biliary dilation, bile duct wall thickening. MRCP: beading and strictures of intrahepatic bile ducts (resembling PSC). Liver biopsy: periductal fibrosis, bile duct damage, Cryptosporidium organisms in biliary epithelium
Management	(1) Eradicate Cryptosporidium (nitazoxanide ± azithromycin — often fails without immune reconstitution). (2) UDCA (ursodeoxycholic acid) 10–20 mg/kg/day — may slow progression (limited evidence). (3) HSCT for immune reconstitution — must be done before advanced cholangitis develops. (4) Liver transplant if end-stage liver disease (may need combined HSCT + liver transplant)
Prognosis	This is the single leading cause of death in HIGM Type 1. Median survival without HSCT is approximately 25 years, with most deaths attributable to liver disease

Critical Clinical Pearl — The Race Against Time in HIGM Type 1

The central management dilemma in HIGM Type 1 is the race between HSCT and Cryptosporidium cholangitis:

If HSCT is performed early (before Cryptosporidium infection or before cholangitis is established) → excellent chance of immune reconstitution and prevention of liver disease
If Cryptosporidium cholangitis develops before HSCT → liver damage is often irreversible and HSCT-related mortality increases significantly due to conditioning regimen hepatotoxicity
This is THE reason early HSCT (ideally before age 2–5 years) is so strongly advocated in Type 1

Complication	Detail
Viral hepatitis	CMV hepatitis, adenovirus hepatitis. Due to defective antiviral immunity (impaired CD8+ T-cell priming)
Hepatocellular carcinoma (HCC)	Chronic liver inflammation → dysplasia → HCC. Reported in HIGM patients with longstanding cholangiopathy
Cholangiocarcinoma	Chronic sclerosing cholangitis → malignant transformation of biliary epithelium (same pathogenesis as PSC-associated cholangiocarcinoma in adults)

3. Pulmonary Complications

Feature	Detail
Subtypes affected	All types
Pathogenesis	Recurrent lower respiratory tract infections (pneumonia, bronchitis) → chronic airway inflammation → destruction of bronchial wall elastic tissue and smooth muscle → irreversible dilation of bronchi. The process is a vicious cycle: damaged airways have impaired mucociliary clearance → retained secretions → further infection → more damage
Clinical features	Chronic productive cough (often purulent sputum), recurrent febrile episodes, digital clubbing (in advanced disease), localised or generalised coarse crackles on auscultation
Investigations	HRCT chest (gold standard): signet-ring sign (dilated bronchus larger than its accompanying pulmonary artery), tram-track lines, bronchial wall thickening, mucus plugging. Lung function tests (spirometry): obstructive pattern
Management	(1) Optimise IgG replacement (target trough > 800 mg/dL to reduce infection frequency). (2) Daily airway clearance / chest physiotherapy. (3) Prompt antibiotic treatment of acute exacerbations (guided by sputum culture). (4) Long-term azithromycin (immunomodulatory/anti-inflammatory; reduces exacerbation frequency). (5) Annual or biennial HRCT for surveillance
Prognosis	Once established, bronchiectasis is irreversible. The goal is to prevent progression through infection control and airway clearance

Feature	Detail
Subtypes affected	Primarily Type 2 (AID deficiency) — where lymphoid hyperplasia is prominent
Pathogenesis	Lymphoid infiltration of the pulmonary interstitium → interstitial inflammation → restrictive lung disease. Related to the generalised lymphoproliferative tendency in AID deficiency
Management	Supportive; may require corticosteroids or immunosuppression if symptomatic

4. Autoimmune and Immune Dysregulation Complications

Autoimmunity is a paradoxical but well-recognised complication of immunodeficiency. In HIGM, the mechanism relates to:

Immune dysregulation: the same pathways that enable class switching also participate in B-cell tolerance. Failure of CSR leads to accumulation of IgM-producing B cells, some of which are autoreactive
Excess IgM autoantibodies: without class switching, the immune system is "stuck" producing IgM. Some of this IgM recognises self-antigens → autoimmune cytopenias
Defective regulatory T-cell function: CD40L–CD40 interactions are involved in Treg homeostasis; disruption contributes to loss of peripheral tolerance

Complication	Mechanism	Clinical Features	Management
Autoimmune haemolytic anaemia (AIHA)	IgM (or IgG) autoantibodies against red blood cell surface antigens → complement-mediated or extravascular haemolysis	Pallor, jaundice, dark urine (haemoglobinuria), fatigue, splenomegaly. Lab: anaemia, ↑reticulocytes, ↑LDH, ↓haptoglobin, positive DAT (Coombs test)	Corticosteroids (1–2 mg/kg/day prednisolone); IVIG (Fc receptor blockade); rituximab if refractory; mycophenolate or sirolimus as steroid-sparing agents
Immune thrombocytopenia (ITP)	IgM autoantibodies against platelet surface glycoproteins (GPIIb/IIIa, GPIb/IX) → splenic phagocytosis of opsonised platelets	Petechiae, purpura, mucosal bleeding, easy bruising	Corticosteroids; IVIG; consider rituximab. Avoid splenectomy if possible (immunodeficient patient → further infection risk)
Autoimmune neutropenia	Anti-neutrophil antibodies (IgM class) → peripheral neutrophil destruction	Oral ulcers, gingivitis, recurrent bacterial infections	G-CSF; IVIG may help. Often overlaps with the cyclical neutropenia seen in Type 1

Immunodeficiency = not just infection susceptible, also increased risk for autoimmune diseases, malignancy etc. ^[2]

Why Autoimmunity in Immunodeficiency? — The Paradox Explained

This seems counterintuitive: how can a patient with a defective immune system have autoimmunity? The answer is that immune regulation and immune effector function are different things. In HIGM:

B-cell tolerance mechanisms (germinal centre selection, receptor editing) require CSR and SHM — both are impaired → autoreactive B cells escape deletion
Regulatory T cells (Tregs) depend partly on CD40L–CD40 interactions for their development and function — disrupted in Types 1/3
Chronic infection itself promotes immune activation and breakdown of tolerance (molecular mimicry, bystander activation) Therefore, immunodeficiency and autoimmunity coexist — a theme seen across many PIDs (CVID, WAS, ALPS, IPEX, etc.)

Manifestation	Details
Inflammatory arthritis	Joint pain, swelling — can resemble juvenile idiopathic arthritis (JIA)
Inflammatory bowel disease (IBD)-like	Chronic diarrhoea with mucosal inflammation (often Cryptosporidium-independent), Crohn-like features
Oral ulceration / serositis	Recurrent aphthous ulcers (partly neutropenia-related, partly autoimmune)
Autoimmune hepatitis	Elevated transaminases, positive autoantibodies; may coexist with/mimic Cryptosporidium cholangitis — careful differentiation needed

Malignancy	Subtypes Affected	Mechanism	Specifics
Non-Hodgkin lymphoma (NHL)	All types, especially Type 1	Chronic immune stimulation + immune dysregulation → lymphoproliferation → malignant transformation. Defective immune surveillance (impaired T-cell help) allows escape of pre-malignant clones. EBV may drive some lymphomas (impaired EBV-specific CD8+ T-cell responses in Type 1)	B-cell NHL is most common; can occur in childhood or adolescence
Cholangiocarcinoma	Type 1, 3	Chronic Cryptosporidium-driven sclerosing cholangitis → chronic biliary inflammation → dysplasia → malignant transformation of biliary epithelium. Same mechanism as PSC-associated cholangiocarcinoma	Rare but devastating; difficult to treat
Hepatocellular carcinoma (HCC)	Type 1, 3	Chronic liver inflammation/cirrhosis from cholangiopathy	Late complication of chronic liver disease
Other solid tumours	Reported but less characterised	Reduced immune surveillance	Case reports; less well-established

Autoimmune cytopenias and/or granulomatous disease and lymphoproliferation are noted as features of CVID phenotype — similar immune dysregulation complications occur in HIGM ^[3]

Complication	Detail
Subtypes affected	Primarily Type 1 (~50% of patients)
Mechanism	Multifactorial: (1) anti-neutrophil IgM autoantibodies, (2) impaired granulopoiesis (CD40L–CD40 may play a role in marrow stromal cell support of myelopoiesis), (3) possible cyclical marrow suppression
Pattern	May be cyclical (neutrophil count fluctuates with ~21-day periodicity, similar to cyclical neutropenia) or chronic
Clinical consequences	Oral ulcers (aphthous stomatitis), gingivitis, periodontitis → poor dentition. Perianal abscesses. Skin infections (cellulitis, furunculosis). Bacterial sepsis during nadir periods
Management	G-CSF (filgrastim); IVIG may help if autoantibody-mediated; good oral hygiene with regular dental care

Complication	Mechanism	Management
Failure to thrive (FTT)	Chronic infections → increased metabolic demand; chronic diarrhoea (Cryptosporidium, Giardia) → malabsorption; anorexia from chronic illness	Optimise infection control, IgG replacement, nutritional support (high-calorie supplements, dietitian input). Treat specific infections
Short stature	Chronic disease, malabsorption, chronic corticosteroid use (for autoimmune complications)	Growth monitoring at every clinic visit; minimise steroid exposure where possible; nutritional optimisation
Psychosocial impact	Chronic hospital visits, school absence, social isolation, anxiety/depression in the child and family	Multidisciplinary support: psychology, social work, school liaison, patient support groups

Complication	Mechanism	Management
Infusion reactions	Fever, chills, headache, myalgia, nausea — due to cytokine release from complement activation by immunoglobulin aggregates	Slow infusion rate; premedicate with paracetamol ± antihistamine; use low-IgA preparations if IgA antibodies present
Aseptic meningitis	Rare; mechanism unclear (possibly immune complex-mediated meningeal inflammation)	Usually self-limiting; supportive care
Thromboembolic events	Increased blood viscosity from high-dose immunoglobulin infusions	More common in adults; ensure adequate hydration; avoid high infusion rates
Haemolysis	Anti-A or anti-B isoagglutinins in IVIG preparations → haemolysis in non-group-O recipients	Monitor Hb post-infusion; use preparations with low isoagglutinin titres

Complication	Timing	Detail
Graft-versus-host disease (GVHD)	Acute: < 100 days post-HSCT; Chronic: > 100 days	Donor T cells recognise recipient tissues as foreign → immune attack on skin, liver, GI tract. Acute: maculopapular rash, cholestatic hepatitis, diarrhoea. Chronic: sclerodermatous skin changes, sicca syndrome, bronchiolitis obliterans
Graft failure/rejection	Early post-HSCT	Donor cells fail to engraft or are rejected by residual host immunity → pancytopenia, absent immune reconstitution
Infections during engraftment	Days 0–30+ post-HSCT	Conditioning-induced neutropenia and immunosuppression → bacterial, fungal, viral infections. CMV reactivation is particularly important to monitor
Veno-occlusive disease (VOD) of liver	Early	Conditioning damages hepatic venous endothelium → hepatomegaly, ascites, jaundice ± fulminant failure ^[10]. Especially dangerous in HIGM patients who may already have pre-existing liver disease from Cryptosporidium cholangitis
Post-transplant lymphoproliferative disease (PTLD)	Late (> 1 year)	EBV-related B-cell lymphoma ^[10]. Due to immunosuppression allowing unchecked EBV-driven B-cell proliferation
Late endocrine effects	Late	Hypothyroidism, hypogonadism, infertility, growth impairment (from TBI or busulfan-based conditioning)

HSCT complications: Early — cytopenia-related (bleeding, neutropenic infections), oral/non-oral mucositis, VOD, acute GVHD. Late — cardiovascular disease, endocrine dysfunction, second malignancy, PTLD, chronic GVHD ^[10]

Complication	Type 1 (CD40L)	Type 2 (AID)	Type 3 (CD40)	Type 5 (UNG)	NEMO
PJP	+++	−	+++	−	++
Cryptosporidium cholangitis	+++ (leading cause of death)	−	+++	−	+
Recurrent bacterial infections	++	++	++	++	++
Bronchiectasis	++	++	++	+	++
Autoimmune cytopenias	++	++	++	+	+
Neutropenia	+++ (~50%)	+ (rare)	+	+ (rare)	+
Lymphoproliferation	+	+++ (massive lymphoid hyperplasia)	+	+	+
NHL / lymphoma	++	++	++	+	++
Hepatobiliary CA	++	−	++	−	+
FTT	++	+	++	+	++
Disseminated BCG (HK)	+++	−	+++	−	+++

Key: +++ = major/hallmark complication; ++ = common; + = possible/reported; − = not typical

Scenario	Median Survival	Key Determinants
HIGM Type 1 without HSCT	~25 years (historically; improving with modern supportive care)	Cryptosporidium cholangitis is the leading cause of death, followed by lymphoma and chronic lung disease
HIGM Type 1 with early HSCT (MSD)	~70–80% long-term survival	Best when transplanted before Cryptosporidium infection and before significant organ damage
HIGM Type 1 with late HSCT or MUD	~50–60% survival	Higher TRM due to pre-existing organ damage, higher GVHD risk
HIGM Type 2 (AID deficiency)	Better (predominantly humoral defect)	Most patients do well on lifelong IgG replacement; lymphoproliferation requires surveillance
HIGM Type 3	Similar to Type 1	Same opportunistic infection risk

High Yield Summary — Complications of HIGM

Leading cause of death in HIGM Type 1: Cryptosporidium sclerosing cholangitis → progressive liver failure / hepatobiliary malignancy. Irreversible once established — drives the urgency for early HSCT.
PJP is often the presenting illness — always do an immunological workup when an infant presents with PJP.
Bronchiectasis develops from recurrent sinopulmonary infections in all subtypes — prevent by optimising IgG trough levels ( > 800 mg/dL) and prompt antibiotic treatment.
Autoimmune cytopenias (AIHA, ITP, autoimmune neutropenia) are paradoxically common — due to defective B-cell tolerance and excess autoreactive IgM.
Neutropenia affects ~50% of Type 1 → oral ulcers, gingivitis, bacterial infections → manage with G-CSF.
Malignancy (NHL, cholangiocarcinoma, HCC) — increased risk due to chronic immune stimulation, defective surveillance, and EBV-driven lymphoproliferation.
Disseminated BCG — a presenting feature in Hong Kong (BCG given at birth) for Types 1, 3, NEMO.
HSCT complications: GVHD, graft failure, VOD (especially dangerous if pre-existing liver disease), infections, PTLD, late endocrine effects.

Active Recall - Complications of Hyper-IgM Syndromes

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 410 — Primary Immunodeficiency, sinopulmonary and GI infections, encapsulated bacteria, Giardia, enterovirus) ^[2] Senior notes: Jerry's immunodeficiencies.pdf (p. 2 — Immunodeficiency not just infection susceptible, also increased risk for autoimmune diseases, malignancy; treatment modalities) ^[3] Lecture slides: Investigations of Imm Disorders 2025.pdf (p. 21 — CVID phenotype features including autoimmune cytopenias, lymphoproliferation, granulomatous disease) ^[10] Senior notes: Ryan Ho Haemtology.pdf (p. 156 — HSCT complications: early and late, VOD, GVHD, PTLD, infections, endocrine dysfunction, second malignancy)

High Yield Summary

Hyper-IgM Syndromes — Key Points for Exams:

Definition: Group of PIDs characterised by ↑IgM, ↓↓IgG/IgA/IgE due to failure of class-switch recombination
Most common subtype: HIGM Type 1 (CD40L deficiency) — X-linked recessive, ~65–70% of cases
Core pathophysiology: CD40L–CD40 interaction is needed for (a) B-cell class switching and (b) macrophage activation → its absence causes both antibody deficiency AND susceptibility to opportunistic infections (PJP, Cryptosporidium)
Why opportunistic infections in Type 1/3 but not Type 2?: CD40L and CD40 are involved in T-cell–macrophage communication; AID is purely intrinsic to B-cell CSR
Key diagnostic test: Flow cytometry for CD40L expression on activated T cells (absent in Type 1); genetic sequencing is confirmatory for all subtypes
Hallmark Ix pattern: ↑IgM, ↓IgG/IgA/IgE, B cells present (CD19+), normal T cell counts (unlike SCID)
Major complications: Cryptosporidium sclerosing cholangitis (leading cause of death in Type 1), autoimmune cytopenias, malignancy, bronchiectasis
Neutropenia occurs in ~50% of Type 1 → oral ulcers, gingivitis
Tonsil clue: Small/absent in Type 1; enlarged in Type 2 (AID deficiency — B cells accumulate in germinal centres)
Curative treatment: HSCT (haematopoietic stem cell transplantation) — especially for Type 1/3 with opportunistic infections; IgG replacement is a lifelong supportive measure
Hong Kong relevance: BCG given at birth → disseminated BCG can be a presenting feature in undiagnosed HIGM Type 1/3/NEMO deficiency

High Yield Summary — Diagnostic Approach to HIGM

The diagnostic algorithm in 6 steps:

Screen: CBC/D (neutropenia?) + Immunoglobulin profile (↑IgM, ↓IgG/IgA/IgE) ^[9]
Exclude secondary causes: HIV, drugs, protein loss, congenital infection ^[6]
Lymphocyte subsets: B cells present (excludes XLA), T cells normal (excludes SCID) ^[6]^[9]
Specific functional test: CD40L expression on activated T cells (Type 1) or CD40 on B cells (Type 3) ^[3]^[6]
Genetic confirmation: Sequencing of CD40LG, AICDA, CD40, UNG, or IKBKG — definitive for all subtypes ^[3]
Complication screening: Cryptosporidium (stool PCR), liver (LFT + USS/MRCP), lungs (HRCT), autoimmune cytopenias (DAT)

Key exam point: "Many IEIs require tailored investigations, and cannot be picked up by initial screening tests. Always consult immunology when clinically suspicious." ^[9]

High Yield Summary — Management of HIGM

All subtypes: Lifelong IgG replacement (IVIG or SCIG) — target trough IgG > 500–800 mg/dL
Types 1, 3, NEMO: Additional co-trimoxazole PJP prophylaxis (check G6PD first!), water precautions for Cryptosporidium, antifungal prophylaxis if recurrent candidiasis, G-CSF if neutropenic
Curative: HSCT — strongly recommended for Types 1, 3, NEMO. Best outcomes when done early (before age 2–5 years), before irreversible Cryptosporidium cholangitis develops
Live vaccines are CONTRAINDICATED in all subtypes; household contacts should be fully vaccinated (cocoon protection)
No effective drug prophylaxis for Cryptosporidium — water safety is the key prevention strategy
Genetic counselling is essential: carrier detection in female relatives (X-linked forms), prenatal diagnosis for future pregnancies
Complication surveillance: LFTs + GGT (cholangiopathy), stool Cryptosporidium PCR, HRCT (bronchiectasis), DAT (autoimmune cytopenias), growth monitoring

High Yield Summary — Complications of HIGM

Leading cause of death in HIGM Type 1: Cryptosporidium sclerosing cholangitis → progressive liver failure / hepatobiliary malignancy. Irreversible once established — drives the urgency for early HSCT.
PJP is often the presenting illness — always do an immunological workup when an infant presents with PJP.
Bronchiectasis develops from recurrent sinopulmonary infections in all subtypes — prevent by optimising IgG trough levels ( > 800 mg/dL) and prompt antibiotic treatment.
Autoimmune cytopenias (AIHA, ITP, autoimmune neutropenia) are paradoxically common — due to defective B-cell tolerance and excess autoreactive IgM.
Neutropenia affects ~50% of Type 1 → oral ulcers, gingivitis, bacterial infections → manage with G-CSF.
Malignancy (NHL, cholangiocarcinoma, HCC) — increased risk due to chronic immune stimulation, defective surveillance, and EBV-driven lymphoproliferation.
Disseminated BCG — a presenting feature in Hong Kong (BCG given at birth) for Types 1, 3, NEMO.
HSCT complications: GVHD, graft failure, VOD (especially dangerous if pre-existing liver disease), infections, PTLD, late endocrine effects.

Hyper-igm Syndromes

1. Definition

2. Epidemiology and Risk Factors

Incidence

Age at Presentation

Risk Factors

3. Anatomy and Function: The Class-Switch Recombination Machinery

4. Aetiology and Classification

4.1 Classification of HIGM Syndromes

4.2 Focus on the Most Important Subtypes

HIGM Type 1 (CD40L Deficiency) — The "Classic" X-Linked Hyper-IgM Syndrome

HIGM Type 2 (AID Deficiency)

HIGM Type 3 (CD40 Deficiency)

NEMO Deficiency (Ectodermal Dysplasia with Immunodeficiency)

5. Pathophysiology Summary Diagram

6. Clinical Features

6.1 General Presentation

6.2 Symptoms

6.3 Signs

6.4 Complications Previewed (detailed management and complications in a later section)

6.5 Infections by Pathogen and Subtype

7. Investigations — Approach to Diagnosis

7.1 First-Line Investigations (Screening)

7.2 Confirmatory / Specific Investigations

7.3 Additional Investigations to Assess Complications

7.4 Distinguishing HIGM from Other Antibody Deficiencies

8. Relevant Paediatric Considerations

8.1 Age-Appropriate Normal Values

8.2 Newborn Screening

8.3 Family-Centred Care and Communication

8.4 Consent and Assent

9. Hong Kong–Specific Considerations

10. Key Differentiating Features Between HIGM Subtypes (Summary Table)

Active Recall - Hyper-IgM Syndromes

References

1. Approach to the Differential — Decision Mermaid Diagram

2. Differential Diagnosis — Primary Immunodeficiencies

2.1 X-linked Agammaglobulinaemia (XLA / Bruton Disease)

2.2 Common Variable Immunodeficiency (CVID)

2.3 Severe Combined Immunodeficiency (SCID)

2.4 Wiskott-Aldrich Syndrome (WAS)

2.5 Hyper-IgE Syndrome (HIES / Job Syndrome)

2.6 DiGeorge Syndrome (22q11.2 Deletion)

2.7 Chronic Granulomatous Disease (CGD)

2.8 Selective IgA Deficiency

3. Differential Diagnosis — Secondary (Acquired) Causes

4. Differentiating Between HIGM Subtypes — Clinicopathological Clues

5. Summary Table — Key Differentials at a Glance

6. Diagnostic Approach — Putting It Together

Active Recall - Differential Diagnosis of Hyper-IgM Syndromes

1. Diagnostic Criteria

1.1 Clinical Diagnostic Framework for HIGM

1.2 Why There Are No "Formal Criteria" Like CVID

2. Diagnostic Algorithm

3. Investigation Modalities — Detailed Breakdown

3.1 STEP 1: First-Line Screening Investigations

3.1.1 Complete Blood Count with Differential (CBC/D)

3.1.2 Serum Immunoglobulin Profile (IgG, IgA, IgM, IgE)

3.1.3 Additional First-Line Tests

3.2 STEP 2: Exclude Secondary Causes

3.3 STEP 3: Second-Line — Lymphocyte Subsets by Flow Cytometry

3.4 STEP 4: Specific Functional / Phenotypic Tests (The Diagnostic Key)

3.4.1 CD40L Expression on Activated T Cells (Flow Cytometry)

3.4.2 CD40 Expression on B Cells

3.4.3 In Vitro Class-Switch Recombination Assay

3.4.4 Vaccine Antibody Responses

3.4.5 Switched Memory B Cells

3.5 STEP 5: Genetic Confirmation (Definitive Diagnosis)

3.6 STEP 6: Complication Screening

4. Investigation Interpretation — Putting It All Together (Summary Table)

5. Special Considerations in Paediatric Investigation

5.1 Age at Testing

5.2 Newborn Screening Limitations

5.3 Live Vaccines and Diagnostic Testing

5.4 Consent for Genetic Testing

Active Recall - Diagnosis and Investigations of Hyper-IgM Syndromes

1. Principles of Management

2. Management Algorithm

3. Treatment Modalities

3.1 Immunoglobulin Replacement Therapy