Where do T lymphocytes mature?

• A. Bone marrow
• B. Thymus
• C. Spleen
• D. Lymph node

Answer: (B)

T lymphocyte maturation occurs in the thymus, where precursor cells from the bone marrow complete their development and undergo selection to ensure they recognize self-MSH molecules with appropriate affinity and do not react strongly to self. The cortex supports positive selection, teaching thymocytes to recognize self-MHC, while the medulla handles negative selection to weed out self-reactive cells. The surviving cells become naive T cells that enter the circulation and migrate to secondary lymphoid organs to encounter antigens. In contrast, B lymphocytes mature in the bone marrow, and the spleen and lymph nodes are sites for antigen encounter and immune activation, not maturation. The thymus is especially prominent in children and gradually involutes with age, though it can continue to produce T cells for some time.