Variable receptors controlling activation and inhibition of NK cells.

NK cells are important effector lymphocytes of innate immunity; they kill infected cells and produce cytokines that stimulate other immune effects. Once considered relatively homogeneous, NK cells are now seen to be highly diverse. Within an individual, expression of different combinations of inhibitory and stimulatory receptors creates a diverse NK cell repertoire, which exhibits specificity in the immune response. Rapid evolution of NK cell receptor gene families distinguishes members of a species and causes substantial species-specific differences in NK cell receptor systems. All known ligands for these diverse receptors are MHC class I molecules, or molecules of host or pathogen origin that are homologous to MHC class I.