Natural killer cell
Natural Killer Cells (NK Cells) are white blood cells and are part of the innate immune response. They contain cytotoxic granules containing enzymes which aid host rejection of tumours and virus-infected cells. They are activated by IFNα, IFNβ, IL12 and produce IFNγ to control infection while the adaptive response begins.
NK Cells have inhibitory and activating receptors on their cell surface. Inhibitory receptors recognise MHC 1 receptors on host cells and this "switches off" the NK Cell, preventing it from killing. The reason that NK Cells can kill cancer cells and infected cells is because they often lose their MHC 1, leaving them vulnerable to NK Cell killing.