NATURAL CYTOXICITY NK Cells Cytotoxicity NK cell inhibitory receptors (IR) and activating receptors (AR) are a very complex group of receptors, which recruit opposing signaling motifs to upregulate or downregulate the activation of NK cells to trigger apoptosis in target cells. The target cells for NK cells are (1) cells infected by viruses, (2) tumor cells, and (3) aging and injured cells of the body. The missing "self" hypothesis suggests that IRs recognize Class I HLA molecules and cancel apoptosis in a target cell, whereas if Class I HLA molecules are not expressed on the surface of the cell, ARs trigger apoptosis in the cell. NK cells are involved in the first phase of the interaction of the immune system and cancer performing apoptosis in cancerous cells. ©V.V.Klimov