• Lymphocyte member of innate immune system.
• Expresses either CD16 or CD56.
• Destroys cells that are infected with viruses or mutated, with removal of MHC class I proteins in their cell surface
• Does not require the thymus for maturation, present in athymic patients
• Use perforin and granzymes to induce apoptosis of virally infected cells and tumor cells.
• Activity enhanced by IL-2, IL-12, IFN-α, IFN-β, IFN-g.
• Induced to kill when exposed to a nonspecific activation signal on target cell and/or to an absence of MHC I on target cell surface.
• No antigen-specific activities, do not require exposure to antigen for activation, no antigen memory ability
• Also kills via antibody-dependent cell-mediated cytotoxicity (CD16 binds Fc region of bound Ig, activating the NK cell).