FcRIreceptor

Fc gamma receptor I (FcγRI), also known as CD64, is a high-affinity receptor for the Fc region of immunoglobulin G (IgG). It is the type I Fc receptor in humans and is unique among Fc receptors for its ability to bind monomeric IgG with high affinity, in contrast to FcγRII and FcγRIII, which preferentially engage multimeric immune complexes.

Structure and expression: FcγRI consists of an α-chain (CD64) that binds the Fc region of IgG and associated signaling γ-chain(s) necessary for activation. The receptor is constitutively expressed on cells of the myeloid lineage, including monocytes, macrophages, and dendritic cells, with variable expression on neutrophils and other cells. Expression can be upregulated in inflammatory states by cytokines such as GM-CSF and IFN-γ.

Ligands and binding: The receptor binds IgG subclasses with a preference for IgG1 and IgG3, while binding IgG2 and IgG4 more weakly. Its high affinity allows binding of monomeric IgG in serum, and cross-linking by immune complexes enhances signaling.

Function and signaling: Cross-linking FcγRI by opsonized pathogens or IgG-coated particles triggers phagocytosis, antigen presentation, and the release of inflammatory mediators. Signaling relies on ITAMs in the associated γ-chain and downstream kinases such as Syk and PI3K, leading to cell activation and pathogen clearance.

Clinical relevance: FcγRI expression increases during infection and inflammation and can serve as a marker of myeloid cell activation (for example, CD64 upregulation on neutrophils). It contributes to host defense through opsonophagocytosis and can participate in immune complex–mediated pathology.