Killer-cell Immunoglobulin-like Receptors
Natural Killer cells (NK) are lymphocytes that can kill tumor cells or virus-infected cells without requiring antibodies. NK cells express both activating and inhibitory receptors on their surface, and the function of NK cells is regulated by the delicate signalling balance between these two types of receptors. Killer-cell immunoglobulin-like receptors (KIRs), are a family of type I transmembrane glycoproteins expressed on the plasma membrane of NK cells and a minority of T cells. They regulate the killing function of these cells by interacting with major histocompatibility (MHC) class I molecules, which are expressed on all nucleated cell types. KIR receptors can distinguish between major histocompatibility (MHC) class I allelic variants, which allows them to detect virally infected cells or transformed cells. Most KIRs are inhibitory, meaning that their recognition of MHC molecules suppresses the cytotoxic activity of their NK cell. Only a limited number of KIRs are activating, meaning that their recognition of MHC molecules activates the cytotoxic activity of their cell. As a result of KIR's role in killing unhealthy self-cells and not killing healthy self-cells, KIRs are involved in protection against and propensity to viral infection, autoimmune disease, and cancer.