FIGURE 23-6 Natural killer cells. Natural killer (NK) cells are an important source of the cytokine interferon γ (IFN-γ), which is involved in antiviral defenses, and can kill virus-infected and cancerous cells directly by means of perforins. These pore-forming proteins allow access to the cytoplasm of the target cell by serine proteases called granzymes. Granzymes can also initiate apoptosis through activation of caspases (see Chapter 21). Receptors on NK cells identify infected or stressed cells and stimulate the NK cell to kill them. Other receptors identify normal cells and inhibit NK cell activation.