FcRI

FcRI, in most immunology contexts, refers to the high-affinity receptor for the Fc portion of immunoglobulin G, commonly designated Fc gamma RI (FcγRI) and CD64. It is encoded by the FCGR1A, FCGR1B, and FCGR1C gene family in humans. The receptor binds monomeric IgG with high affinity and is expressed chiefly on myeloid cells such as monocytes and macrophages, with inducible expression on neutrophils and dendritic cells during inflammation. It plays a key role in opsonization, phagocytosis, and antibody-dependent cellular cytotoxicity, by capturing IgG-coated pathogens and triggering intracellular signaling through the γ-chain ITAM motifs after receptor cross-linking.

Structurally, FcγRI consists of an α-chain that binds the Fc region of IgG and two associated γ-chain dimers that mediate intracellular signaling. This configuration enables rapid phagocytosis and activation of inflammatory pathways when immune complexes engage the receptor. Expression is upregulated by cytokines such as IFN-γ, GM-CSF, and G-CSF, which enhances the phagocytic and microbicidal capacity of target cells during infection or inflammation.

Clinically, FcγRI is used as a cellular marker in flow cytometry (CD64) to identify monocytes and activated neutrophils. Elevated neutrophil CD64 levels can serve as a biomarker for bacterial infection and sepsis. Genetic variation in FCGR1 genes can influence receptor expression and affinity, affecting immune complex handling and susceptibility to autoimmune or infectious diseases.

Note: FcRI is sometimes used informally to refer to other Fc receptors, such as the high-affinity IgE receptor FcεRI. In standard nomenclature, FcγRI denotes the receptor for IgG.