Natural killer (NK) cells express cell surface receptors that recognize major histocompatibility complex class I peptides (MIM.142800) and inhibit NK cell-mediated cytotoxicity.
NK cell receptors belong to 2 distinct groups: the immunoglobulin superfamily for the killer cell inhibitory receptors (KIRs) (MIM.602992), and the C-type lectin superfamily for the NKG2 receptors (MIM.161555).
These inhibitory receptors possess ’immunoreceptor tyrosine-based inhibitory motifs’ (ITIMs) in their cytoplasmic domains that recruit SH2 domain-containing protein-tyrosine phosphatases, resulting in inactivation of NK cells.
Certain isoforms of NK cell receptors lack ITIM sequences, and these non-inhibitory receptors activate, rather than inhibit, NK cells.
Cell surface immunoglobulin receptors, T-cell antigen receptors (MIM.186880), and certain Fc receptors noncovalently associate with small transmembrane proteins, such as CD3-delta (CD3D)(MIM.186790), -gamma (CD3G) (MIM.186740), -epsilon (CD3E) (MIM.186830), -zeta (CD3Z) (MIM.186780), CD79-alpha (MIM.112205), -beta (MIM.147245), and FCER1G (MIM.147139), that contain an ’immunoreceptor tyrosine-based activation motif’ (ITAM) sequence and are required for signal transduction by these receptor complexes.