CD89

CD89, or Fc alpha receptor I (FcαRI), is a receptor for the Fc portion of immunoglobulin A (IgA). It is encoded by the FCAR gene in humans and belongs to the Fc receptor (FcR) family. CD89 is a type I transmembrane protein that forms a signaling complex with the FcRγ chain, which provides ITAM-based signaling.

CD89 is expressed primarily on myeloid cells, including neutrophils, monocytes, eosinophils, and dendritic cells, in blood and various tissues. It binds both IgA1 and IgA2 with varying affinities and can interact with soluble IgA as well as IgA-containing immune complexes. Engagement of CD89 by IgA–antigen complexes triggers antibody-dependent cellular responses, notably phagocytosis, respiratory burst, degranulation, and the production of inflammatory mediators, facilitating clearance of IgA-coated pathogens at mucosal surfaces and systemically.

Clinical relevance: CD89 plays a role in IgA-mediated immune responses and is implicated in the pathogenesis of IgA nephropathy, where IgA immune complexes may activate CD89-expressing cells and contribute to inflammation. In immunotherapy research, CD89 is of interest as a means to recruit neutrophils for antibody-dependent cellular cytotoxicity using engineered IgA antibodies, offering alternatives to IgG-based approaches.

CD89 is one of several Fc receptors that enable IgA to participate in immune defense, and its function reflects the balance between protective mucosal immunity and inflammation in IgA-related diseases.