·        

4.2.4 Natural killer cells

Many natural killer (NK) cells express the inhibitory receptors that recognizes the MHC class I molecules. Inhibitory receptors contain a unique structure in their cytoplasmic tail called immunoreceptor tyrosine-based inhibition motif (ITIM), which blocks the signaling pathways of the activating receptors. Activating receptor of NK cells contains immunoreceptor tyrosine-based activation motif (ITAM), which promotes the infected cell killing and cytokine secretion. NK cells contains a unique CD molecule called CD16 over their surface that has an affinity towards microbial bound IgG molecules, the phenomenon is called antibody-dependent cell-mediated cytotoxicity .

NK cells recognize the ligands of infected cells or cells undergoing stress and kill the host cells. This helps in the elimination of infection and also unwanted cell population in the human body. IL-12 is produced by the macrophages that phagocytize the microbial antigens, the NK cells secretes interferon-γ in response to IL-12 and kill the phagocytized pathogen.

Figure 4.1 Natural killer cell mediate innate immune response: