Natural Killer (NK) cell (purple, centre) attacking a cancer cell, illustration. NK cells are lymphocytes (white blood cells) that act as part of the body's innate immune response. They have receptors to detect and bind to molecules expressed on the surface of cancerous cells. Once a cancerous cell is detected, NK cells release cytotoxic granules containing enzymes and proteins that kill the cell. NK-92 cell lines have higher cytotoxic activity and are being researched for use in immunotherapies.