Killer cell immunoglobulin-like receptors (KIR or KIR-receptors) are receptors which are mainly present at the plasma membrane of natural killer cells (NK cells). They play a big role in the mammal immune system. NK cells use KIR to detect pathological cells, like tumor or infected cells.

Each individuum just contain a given repertoire of the 15 known KIR genes. Both activating and inhibiting KIR are present, the latter binding to MHC class I, in particular HLA-C proteins.  If the sum of the inhibiting KIR signals are weaker than the activating ones, the target cell is lysed. In bone marrow or stem cell recipients with missing HLA class I determinants, for example, this can lead to the destruction of the tumor cells by the donor's NK cells (GvL graft versus leukemia effect) and thereby reduce the risk of rejection (Dupont und Hsu, 2004).

Combinations of MHC class I and KIR variants not only correlate with the outcome of transplantations, but also with autoimmune (including rheumatoid arthritis, psoriasis) and viral diseases (including HIV, EBV, HCV infection) and pregnancy complications  (Parham, 2005). KIR genotyping is also included in the diagnosis for couples with an unfulfilled desire to have children without any indication of infertility. There is evidence that both male and female KIR genotypes may play a role in IVF success and cause miscarriage  (Nowak et al., 2017;  Wilczyńska et al., 2020).