Natural killer (NK) cell attacking a cell infected by a virus (upper right). Natural killer cells are a type of lymphocyte (white blood cell) and a component of the immune system. The NK cell attaches itself to the target cell and proteins called perforin pierce the cell membrane, allowing cytotoxin granules to enter the cell. The cytotoxin granules expel granzymes which promote cell apoptosis (programmed cell death). Labeled.