Bruton's Agammaglobulinemia vs Common Variable Immunodeficiency
Both Bruton's X-linked agammaglobulinemia (XLA) and common variable immunodeficiency (CVID) are humoral (antibody) deficiencies that present with recurrent sinopulmonary infections by encapsulated bacteria. The core axis that separates them is the B cell: in XLA, a BTK mutation blocks pre-B cell maturation, so B cells and lymphoid tissue are absent, and boys present in infancy; in CVID, B cells are present but fail to differentiate into plasma cells, so patients present in adolescence or adulthood with enlarged lymphoid tissue and autoimmune/malignant complications.
How to tell them apart
| Feature | Bruton's Agammaglobulinemia (XLA) | Common Variable Immunodeficiency (CVID) |
|---|---|---|
| Inheritance | X-linked recessive | Usually sporadic |
| Age of onset | Under 1 year, once maternal antibody wanes (after ~6 months) | Adolescence or adulthood |
| B cells | Absent or profoundly decreased (<1%, CD19+/CD20+ absent) | Present but dysfunctional, with low class-switched memory B cells |
| Lymphoid tissue | Absent — no visible tonsils, tiny/impalpable lymph nodes | Often enlarged (lymphoid hyperplasia) |
| Sex affected | Males only | Both males and females |
| Key molecular defect | BTK mutation — B cells cannot mature from pre-B cells | Heterogeneous defect — B cells fail to differentiate into plasma cells |
| Associated complications | Giardiasis and enteroviral infections; risk from live vaccines | Bronchiectasis, autoimmune disease (~20%), increased risk of lymphoma and gastric cancer |
The reasoning
Anchor first on the B cell count and age/sex. A male infant presenting before age 1 with recurrent encapsulated bacterial infections, absent tonsils/nodes, and near-zero CD19+ B cells with absent immunoglobulins across all classes is Bruton's XLA — the BTK defect prevents B cells from ever maturing, so there is no lymphoid tissue to palpate. In contrast, an adolescent or adult of either sex with recurrent sinopulmonary infections, low IgG and IgA, but present (circulating) B cells and often enlarged lymph nodes points to CVID, where B cells exist but cannot differentiate into plasma cells. The presence versus absence of B cells and lymphoid tissue is the decisive arbitrator; layered onto that, later onset, either-sex involvement, autoimmunity, and malignancy risk all favor CVID.
Key tests
- Flow cytometry for B cells (CD19+/CD20+): absent (<1%) in XLA, present (though dysfunctional, with reduced class-switched memory B cells) in CVID.
- Quantitative serum immunoglobulins: XLA shows absent/profoundly decreased IgG, IgA, and IgM; CVID shows low IgG and IgA with variable IgM.
- Physical examination of lymphoid tissue: absent tonsils and impalpable lymph nodes in XLA versus enlarged lymphoid tissue in CVID.
- Genetic testing for BTK mutation: positive in XLA (X-linked); CVID is typically sporadic without a single defining mutation.
What they share
- Recurrent sinopulmonary infections, classically with encapsulated bacteria
- Low serum immunoglobulins reflecting an antibody deficiency
- Functionally intact humoral response is lost, requiring immunoglobulin replacement rather than being an isolated T-cell problem
Pitfalls
- Both cause recurrent encapsulated bacterial infections and low immunoglobulins, so relying on the infection pattern alone will not separate them — you must check for B cell presence and lymphoid tissue.
- Assuming any male with antibody deficiency has XLA: CVID affects both sexes, and finding circulating B cells or enlarged nodes should redirect you to CVID.
- Forgetting that in CVID B cells are present but dysfunctional — a normal B cell count does not exclude a serious humoral deficiency.
- Overlooking CVID's long-term complications (autoimmune cytopenias, bronchiectasis, lymphoma, gastric cancer), which are not features of XLA.
- Missing that XLA presents only after maternal IgG wanes (~6–9 months), so a newborn may appear deceptively well in the first months of life.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.