Which statement about natural killer (NK) cells is true?

• A. Does not require previous exposure to an antigen
• B. Involves phagocytosis and killing of bacteria
• C. Requires interaction with cytotoxic T cells
• D. Requires interaction with B cells

Answer: (A)

Natural killer cells are part of the innate immune system and provide rapid defense against virally infected or transformed cells without needing prior exposure to a specific antigen. They recognize stressed or abnormal cells partly by reduced MHC I expression and by activating signals, then kill these targets using perforin and granzymes. They can also mediate antibody-dependent cellular cytotoxicity when antibodies coat a target, via the NK cell’s Fc receptor, but this does not require the NK cells to have seen the antigen before.

So the statement that they do not require previous exposure to an antigen is true. They are not primarily phagocytes, so they don’t kill by engulfing bacteria; they don’t need to interact with cytotoxic T cells or with B cells to perform their main function.