What is the role of natural killer (NK) cells?

• A. They produce antibodies against antigens
• B. They provide direct cytotoxic effects on non-self cells without prior sensitization
• C. They are the primary phagocytes
• D. They regulate T cell activity only

Answer: (B)

Natural killer cells act in innate immunity by killing target cells without prior sensitization. They recognize stressed, virus-infected, or tumor cells that often display reduced or altered MHC class I molecules or other activating signals, and then release cytotoxic granules containing perforin and granzymes to induce apoptosis in the target. This allows rapid, nonspecific destruction of abnormal cells early in an infection or tumor development, without the need for antigen-specific learning. NK cells can also mediate antibody-dependent cellular cytotoxicity through CD16, enhancing killing of antibody-coated targets. They contribute cytokines like IFN-γ to boost other immune responses, but their defining role is direct cytotoxic killing rather than antibody production, phagocytosis, or exclusively regulating T cells.