FCAR

Fc fragment of IgA receptor (FCAR) is a human gene that codes for the transmembrane receptor FcαRI, also known as CD89 (Cluster of Differentiation 89). FcαRI binds the heavy-chain constant region of Immunoglobulin A (IgA) antibodies. FcαRI is present on the cell surface of myeloid lineage cells, including neutrophils, monocytes, macrophages, and eosinophils, though it is notably absent from intestinal macrophages and does not appear on mast cells. FcαRI plays a role in both pro- and anti-inflammatory responses depending on the state of IgA bound. Inside-out signaling primes FcαRI in order for it to bind its ligand, while outside-in signaling caused by ligand binding depends on FcαRI association with the Fc receptor gamma chain (FcR γ-chain).

Though FcαRI is part of the Fc receptor immunoglobulin superfamily, the protein's primary structure is similar to receptors in the leukocyte receptor cluster (LRC), and the FCAR gene appears amidst LRC genes on chromosome 19. This contrasts with the location of other members of the Fc receptor immunoglobulin superfamily, which are encoded on chromosome 1. Additionally, though there are equivalents to FCAR in several species, there is no such homolog in mice.