What is the main function of natural killer (NK) cells in innate immunity?

• A. Present antigen to T cells
• B. Phagocytose bacteria
• C. Recognize and kill cells with downregulated MHC I, and mediate ADCC via CD16
• D. Produce antibodies against pathogens

Answer: (C)

Natural killer cells act as early defenders in innate immunity by targeting abnormal cells. They monitor for “missing self” through MHC class I expression: normal cells display MHC I that sends inhibitory signals to NK cells, preventing killing; many virus-infected or transformed cells lose MHC I, removing that inhibition and allowing NK cells to destroy them via cytotoxic granules. They also engage antibody-coated targets through CD16 (a Fc receptor), triggering antibody-dependent cellular cytotoxicity and killing of opsonized cells. This combination—direct killing of cells with downregulated MHC I and ADCC via CD16—captures the primary role of NK cells, unlike presenting antigens, phagocytosis, or antibody production.