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

• A. They phagocytose bacteria.
• B. They kill virus-infected and tumor cells as part of innate immunity.
• C. They produce antibodies.
• D. They present antigens to T cells.

Answer: (B)

Natural killer cells are frontline defenders of the innate immune system. They patrol for cells that are infected or have become cancerous and respond quickly to stop the threat. They recognize trouble signals on stressed or abnormal cells, such as reduced MHC class I presentation common in virally infected or transformed cells. Once they detect a target, NK cells release cytotoxic granules containing perforin and granzymes, which induce apoptosis in the offending cell and help halt virus spread or tumor growth. They can also mediate antibody-dependent cellular cytotoxicity if antibodies are bound to a target, using their Fc receptor to enhance killing. This role fits NK cells as innate killers rather than producers of antibodies, phagocytes, or antigen-presenting cells.