Bruton’s agammaglobulinemia is caused by a mutation in which gene and what is the consequence?

• A. Defect in BTK
• B. Defect in NADPH oxidase
• C. Defect in IL-2 receptor gamma chain
• D. Absence of MHC class II

Answer: (A)

Bruton agammaglobulinemia results from a defect in Bruton tyrosine kinase (BTK), which is essential for B cell development. Without functional BTK, B cells cannot mature beyond the pre-B cell stage in the bone marrow, so very few mature B cells reach the blood. Consequently, immunoglobulins are greatly reduced or absent, leading to poor antibody responses and recurrent bacterial infections after maternal antibodies wane. T cell numbers and function are typically normal, so cellular immunity remains intact. The other listed defects cause different immunodeficiencies (e.g., issues with phagocyte ROS production, severe combined immunodeficiency, or lack of MHC class II), not the B cell maturation block seen in Bruton.