Fc gamma receptor I, II, and III (FcγRI, FcγRII, FcγRIII)

Fc gamma receptors I, II, and III are a family of cell surface receptors expressed primarily on phagocytes such as macrophages, neutrophils, monocytes, and dendritic cells. They bind to the constant region (Fc) of immunoglobulin G (IgG) antibodies that have opsonized pathogens or target cells. This interaction mediates key effector functions including phagocytosis, where opsonized particles are engulfed by phagocytes; antibody-dependent cellular cytotoxicity; endocytosis; and modulation of inflammatory responses[1][3][4]. Each subtype has unique structural features and signaling mechanisms. For example: * FcγRI has high affinity for monomeric IgG. * FcγRIIA contains an intrinsic ITAM motif in its cytoplasmic tail that initiates signaling upon crosslinking. * FcγRIIB is inhibitory due to an ITIM motif. * FcγRIIIA/IIIB require association with a common γ-chain containing ITAMs for signal transduction. Upon engagement by immune complexes or therapeutic antibodies' Fc domains, these receptors trigger intracellular signaling cascades involving tyrosine phosphorylation motifs like ITAMs. This leads to recruitment and activation of kinases such as Syk and PI3K—ultimately resulting in actin remodeling required for engulfment/phagosome formation or triggering cell-mediated killing mechanisms[1][3][5]. These receptors play central roles in host defense against infection but are also implicated in autoimmune diseases when dysregulated. Therapeutically engineered monoclonal antibodies often exploit these pathways to enhance clearance of target cells through improved binding affinity with specific Fc gamma receptor subtypes[2]. Overactivation can lead to adverse inflammatory reactions. "An important function of Fc gamma receptors is the ingestion or phagocytosis of IgG sensitized cells... isoforms from each class can mediate phagocytosis although requirements differ"[1]. "The interaction between Fc-FcγR mediates intracellular protective activity against monoclonal antibody... involvement induces multipotent pro-inflammatory functions"[4].

Other names

CD64 (FcγRI)CD32 (FcγRII)CD16 (FcγRIII)IgG Fc receptor

Engagement of the receptor by the IgG-Fc region triggers phagocytosis or ADCC[1][3][4] Crosslinking leads to ITAM phosphorylation and downstream activation of kinases such as Syk[1][3][5]