In the thymus, which processes occur?

• A. B cell activation
• B. T cell differentiation and maturation
• C. Antibody secretion
• D. Phagocytosis by macrophages

Answer: (B)

The thymus is the site where T cells differentiate and mature. Developing thymocytes come from bone marrow and migrate to the thymus, where they undergo TCR gene rearrangement and maturation. In the cortex, thymic epithelial cells present self-MHC molecules, driving positive selection so only thymocytes that can recognize self-MHC survive. In the medulla, negative selection eliminates thymocytes that bind self-antigens too strongly, promoting self-tolerance. The result is mature T cells that will express either CD4 or CD8 and patrol the body to respond to antigens presented by antigen-presenting cells.

B cell activation and antibody secretion occur mainly in peripheral lymphoid tissues after antigen exposure, and phagocytosis by macrophages is a broader innate function not specific to thymic maturation.