Which type of lymphocyte accounts for about 30% of lymphocytes?

• A. T cells
• B. B cells
• C. Natural Killer cells
• D. Monocytes

Answer: (C)

Natural killer cells are a large, rapid-response lymphocyte subset that provides immediate cytotoxic defense against virally infected and tumor cells. They make up a sizable portion of circulating lymphocytes—about a third in many individuals—which is why they’re described as accounting for roughly 30% of lymphocytes. NK cells operate without needing prior sensitization or antigen presentation, distinguishing them from T cells, which rely on antigen-specific receptors, and from B cells, which mainly produce antibodies. This innate cytotoxic role explains why they’re such a substantial part of the lymphocyte population. Monocytes are a separate, non-lymphoid lineage and are not counted among lymphocytes.