KIRs are human natural killer (NK) cell receptors for MHC class I molecules. Most inhibit the activity of NK cells thus preventing cell lysis. Two main families of receptors for HLA class I have been discovered on human NK cells: Killer cell Immunoglobulin-like receptors (KIR) and CD94/NKG2 heterodimers. The KIR receptors are an extremely diverse family of receptors. In addition to receptors for HLA, NK cells also express pattern recognition receptors e.g. TLRs and Nod proteins.