Which cells are large granular lymphocytes that destroy abnormal cells via non-specific mechanisms?

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

Answer: (C)

Natural killer cells are large granular lymphocytes that act in the innate immune system to destroy abnormal cells without prior sensitization. They recognize stressed or abnormal cells—often via reduced MHC I expression or stress ligands—and release cytotoxic granules containing perforin and granzymes to trigger target cell death. They can also kill targets through antibody-dependent cellular cytotoxicity when antibodies are bound to them, using Fc receptors to enhance killing. This non-specific, rapid response distinguishes NK cells from other lymphocytes. B cells are antibody-producing cells of the adaptive arm, while cytotoxic T cells rely on antigen presentation to recognize and kill infected or abnormal cells. Monocytes are phagocytes, not large granular lymphocytes, and kill mainly by engulfing targets rather than granule-mediated cytotoxicity.