A 20-year-old man has recurrent sino-pulmonary infections, absent tonsils, and profound reduction in all immunoglobulin classes. Flow cytometry shows absent CD19+ B cells but normal T cell numbers. The genetic defect in this condition involves:

• A Adenosine deaminase (ADA) deficiency
• B Bruton's tyrosine kinase (BTK) gene mutation ✓
• C RAG-1 or RAG-2 gene mutations
• D CD40L (CD154) deficiency on T cells

Explanation

X-linked agammaglobulinaemia (Bruton's disease) is caused by mutation in the BTK gene encoding Bruton's tyrosine kinase, which is essential for pre-B cell receptor signalling and B cell maturation. Without functional BTK, B cell development arrests at the pre-B stage, resulting in absent mature B cells (CD19−), absent plasma cells, absent tonsils/lymphoid follicles, and profound panhypogammaglobulinaemia. ADA deficiency causes SCID (affecting both T and B cells); RAG mutations cause T-B-SCID; CD40L deficiency causes Hyper-IgM syndrome.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.