Immunoglobulin (Ig)-like domain found in Leukocyte Ig-like receptors, natural and similar domains
Ig_LILR_KIR_like: domain similar to the first and second immunoglobulin (Ig)-like domains found in Leukocyte Ig-like receptors (LILRs) and Natural killer inhibitory receptors (KIRs). This group includes LILRB1 (or LIR-1), LILRA5 (or LIR9), an activating natural cytotoxicity receptor NKp46, the immune-type receptor glycoprotein VI (GPVI), and the IgA-specific receptor Fc-alphaRI (or CD89). LILRs are a family of immunoreceptors expressed on expressed on T and B cells, on monocytes, dendritic cells, and subgroups of natural killer (NK) cells. The human LILR family contains nine proteins (LILRA1-3,and 5, and LILRB1-5). From functional assays, and as the cytoplasmic domains of various LILRs, for example LILRB1 (LIR-1), LILRB2 (LIR-2), and LILRB3 (LIR-3) contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs) it is thought that LIR proteins are inhibitory receptors. Of the eight LIR family proteins, only LIR-1 (LILRB1), and LIR-2 (LILRB2), show detectable binding to class I MHC molecules; ligands for the other members have yet to be determined. The extracellular portions of the different LIR proteins contain different numbers of Ig-like domains for example, four in the case of LILRB1 (LIR-1), and LILRB2 (LIR-2), and two in the case of LILRB4 (LIR-5). The activating natural cytotoxicity receptor NKp46 is expressed in natural killer cells, and is organized as an extracellular portion having two Ig-like extracellular domains, a transmembrane domain, and a small cytoplasmic portion. GPVI, which also contains two Ig-like domains, participates in the processes of collagen-mediated platelet activation and arterial thrombus formation. Fc-alphaRI is expressed on monocytes, eosinophils, neutrophils and macrophages; it mediates IgA-induced immune effector responses such as phagocytosis, antibody-dependent cell-mediated cytotoxicity and respiratory burst.