Which enzymes do NK cells use to kill their targets?

• A. Granzymes and perforin
• B. Caspases and granzymes
• C. Perforin alone
• D. Cytotoxic granule proteases only

Answer: (A)

Natural killer cells kill by releasing cytotoxic granules that contain perforin and granzymes. Perforin creates pores in the target cell membrane, allowing granzymes to enter the cytosol. Once inside, granzymes—especially granzyme B—activate the caspase cascade and other apoptotic pathways, driving the target cell to die. Caspases themselves are intracellular proteases within the target cell, not released by NK cells, so they aren’t the enzymes NK cells deliver. Perforin alone wouldn’t be enough without the delivered granzymes to trigger apoptosis. Cytotoxic granule proteases without perforin wouldn’t reach the target cell interior. That combination of granzymes and perforin is what NK cells use to kill their targets.