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

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.

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.