Which of the following is the most common form of primary immunodeficiency and is characterized by defective B-cell maturation due to mutations in the BTK gene?

Explanation

## X-Linked Agammaglobulinemia (Bruton's Agammaglobulinemia) ### Genetic Basis **Key Point:** X-linked agammaglobulinemia is caused by mutations in the *BTK* gene (Bruton's tyrosine kinase), located on the X chromosome (Xq21.3–q22). ### Pathophysiology 1. BTK is a non-receptor tyrosine kinase essential for B-cell receptor (BCR) signaling 2. BTK mutations → defective BCR signaling → impaired B-cell maturation and proliferation 3. Result: **Absent or severely reduced B cells** in bone marrow and peripheral blood 4. T-cell numbers and function are **normal** 5. Plasma cells are absent → **agammaglobulinemia** (absent immunoglobulins) ### Clinical Features | Feature | Finding | |---------|----------| | **Age of onset** | 3–6 months (after maternal IgG wanes) | | **Gender** | Males exclusively (X-linked recessive) | | **B-cell count** | <1% of normal (absent or severely reduced) | | **Immunoglobulin levels** | IgG <100 mg/dL, IgA/IgM absent | | **T-cell function** | Normal | | **Infections** | Recurrent pyogenic infections (S. pneumoniae, H. influenzae, S. aureus) | | **Opportunistic infections** | Rare (T-cell immunity preserved) | | **Lymph nodes/spleen** | Absent or hypoplastic | ### Diagnosis **High-Yield:** - Flow cytometry: CD19^+^ B cells <1% (vs. normal 5–15%) - Serum immunoglobulin levels: markedly reduced - BTK gene sequencing: confirms diagnosis ### Management - **Intravenous immunoglobulin (IVIG)** replacement (400–600 mg/kg/month) - Prophylactic antibiotics - Live vaccines contraindicated **Clinical Pearl:** Despite absent B cells, patients do NOT develop opportunistic infections (like PCP, CMV) because T-cell immunity is intact. This distinguishes XLA from SCID. ### Why XLA Is the Answer **Mnemonic:** **BTK = B-cell Tyrosine Kinase** — defects in BTK cause the most common **X-linked** primary immunodeficiency affecting B cells.