Natural killer (NK) cells are a subset of lymphocytes which play a crucial role in early innate immune response against infection and tumor transformation. Furthermore, they secrete interferon-γ (IFN-γ) and tumor necrosis factor (TNF) prompting adaptive immu-nity. NK cells distinguish the unhealthy cells from the healthy ones through an array of cell-surface receptors. Human NK cells use inhibitory and activating killer cell Ig-like receptors (KIR) as primary probe to discriminate between healthy and unhealthy cells. The inhibitory KIRs recognize HLA class I molecules and trigger signals that stop NK killing. The activating KIRs are believed to recognize the determinants associated with infections and tumors, and trigger signals that activate NK killing. Therefore, the effec-tor function of a given NK cell depends upon the receptors that it expresses and ligands that it recognizes on the targets. Genes encoding KIRs and HLA ligands are located on different chromosomes, and vary in number and type. The independent segregation of KIR and HLA genes results in variable KIR-HLA combinations in individuals, which may determine the individual’s immunity and susceptibility to disease.