NKR-P1-A,B,C

Natural Killer (NK) cells are large, granular lymphocytes found primarily in peripheral blood, where they make up about 10% of the lymphocyte population. Their function is to defend against certain types of tumor cells and virally infected cells. When triggered, the cells release cytotoxic granules which induce either apoptosis or necrosis in the target cell. There are known to be two sets of cell surface receptor on NK cells which recognize target cells with opposing functions. One set, of which an example is NKR-P1 receptors, trigger cytokine release and cell cytotoxicity and the other set, of which Ly49A is an example, inhibit cytokine release. These receptors bind with MHC class I molecules on the surface of healthy cells which inhibits the release of cytokines. In virally infected or cancerous cells, class I MHC molecules are not expressed, inducing the release of cytokine. An activation signal is known to exist, however the mechanics of this signal are not very well understood.