Which process drives the specificity and diversity of B and T cell receptors upon antigen exposure?

• A. Somatic mutation during infection
• B. Clonal selection and expansion of lymphocytes upon antigen exposure
• C. Antigen presentation by macrophages
• D. Phagocytosis of pathogens by neutrophils

Answer: (B)

Clonal selection and expansion after antigen exposure drives the targeted, specific response. When an antigen is encountered, only the B and T lymphocytes whose receptors bind that antigen are activated. Those specific clones proliferate (expand) and differentiate into effector and memory cells, producing a strong, tailored response to the antigen. The diversity of B and T receptors is generated earlier by V(D)J recombination during lymphocyte development, while the expansion step focuses on the antigen-specific clones that were already available. Antigen presentation by macrophages helps activate T cells, but it doesn’t create receptor diversity, and phagocytosis by neutrophils belongs to innate defense rather than shaping receptor specificity.