NCIt related terms : Natural killer cell mediated cytotoxicity;
Alternative definition : KEGG: Natural killer (NK) cells are lymphocytes of the innate immune system that are
involved in early defenses against both allogeneic (nonself) cells and autologous
cells undergoing various forms of stress, such as infection with viruses, bacteria,
or parasites or malignant transformation. Although NK cells do not express classical
antigen receptors of the immunoglobulin gene family, such as the antibodies produced
by B cells or the T cell receptor expressed by T cells, they are equipped with various
receptors whose engagement allows them to discriminate between target and nontarget
cells. Activating receptors bind ligands on the target cell surface and trigger NK
cell activation and target cell lysis. However Inhibitory receptors recognize MHC
class I molecules (HLA) and inhibit killing by NK cells by overruling the actions
of the activating receptors. This inhibitory signal is lost when the target cells
do not express MHC class I and perhaps also in cells infected with virus, which might
inhibit MHC class I expression or alter its conformation. The mechanism of NK cell
killing is the same as that used by the cytotoxic T cells generated in an adaptive
immune response; cytotoxic granules are released onto the surface of the bound target
cell, and the effector proteins they contain penetrate the cell membrane and induce
programmed cell death.;