Which lymphocyte types undergo clonal selection and expansion upon antigen exposure?

• A. Red blood cells
• B. B cells and T cells
• C. Natural killer cells
• D. Platelets

Answer: (B)

The idea tested is that antigen exposure drives clonal selection in the adaptive immune system. Lymphocytes that have receptors specific for a given antigen are stimulated to proliferate and differentiate, creating many identical copies (clones) of those antigen-specific cells. This is how the body mounts a targeted and lasting response.

B cells and T cells are the ones that undergo this process. B cells, upon recognizing their specific antigen, proliferate and mature into antibody-secreting plasma cells and memory B cells, providing humoral immunity. T cells, once their T cell receptor matches the antigen, expand into helper and cytotoxic effectors (and memory T cells), coordinating and carrying out cellular immunity.

Natural killer cells are part of the innate immune system and respond without the same kind of antigen-specific receptor–driven clonal expansion. They act more broadly against infected or stressed cells, rather than expanding in a highly specific, clonal manner. Red blood cells and platelets are not immune cells and do not participate in antigen-specific clonal selection.

So, the cells that undergo clonal selection and expansion upon antigen exposure are the B and T lymphocytes.