Which statement about NK cells is true?

• A. Their response against pathogens is very specific.
• B. They only react when an abundance of MHC antigens is present.
• C. They react when both an inhibitory and activating signal is triggered.
• D. They are able to kill target cells without previous exposure to them.

Answer: (D)

Natural killer cells are part of the innate immune system, delivering rapid, nonspecific killing without prior exposure to a specific pathogen. They don’t rely on antigen-specific receptors like T or B cells; instead, they sense cues of cellular stress and changes in MHC class I expression. If a target cell has reduced MHC I or presents stress ligands, activating receptors on NK cells outpace inhibitory signals, triggering release of cytotoxic molecules like perforin and granzymes to destroy the target. They can also kill targets via antibody-dependent cellular cytotoxicity when antibodies coat a cell, engaging CD16. This explains why they can eliminate unfamiliar or transformed cells without previous encounters. The other statements misrepresent NK cell behavior: their activity isn’t highly specific to particular pathogens; they’re not activated merely by abundant MHC antigens (in fact, MHC I presence usually dampens NK activity); and while their decision is based on a balance of signals, it’s not simply that both inhibitory and activating signals must be triggered as a fixed rule—the net balance dictates activation.