Which molecules are released by natural killer (NK) cells that form pores in target cell membranes to cause lysis?

• A. Cytokines
• B. Antibodies
• C. Kinins
• D. Perforins

Answer: (D)

Natural killer cells kill target cells by releasing toxic granules that contain perforin and granzymes. Perforin is the molecule that forms pores in the target cell membrane, creating openings through which granzymes can enter the cytoplasm. Once inside, granzymes activate caspases and other pathways that drive apoptosis of the stressed or abnormal cell. Cytokines are signaling molecules that modulate the immune response but don’t form pores to lyse cells; antibodies are produced by B cells to tag targets, and kinins are inflammatory mediators, not pore-forming lytic proteins.