Fc region of immunoglobulin G antibody (Fc region (IgG))

The **Fc region** (*fragment crystallizable* region) is the tail portion of an immunoglobulin G (**IgG**) antibody. It is composed primarily of the constant domains from two heavy chains. Unlike the variable Fab regions that bind antigens, all IgGs from a given species share an identical sequence in their respective class's **Fc** regions. The main biological role is mediating interactions with cell surface **Fc receptors** on various immune cells—such as macrophages, neutrophils, natural killer cells—and proteins like C1q in the complement system. These interactions enable key effector functions including opsonization, cell lysis via ADCC, degranulation responses in granulocytes, and regulation/extension of serum half-life through binding to neonatal Fc receptor (**FcRn**) which also facilitates placental transfer. The structure includes a highly conserved N-glycosylation site at Asn297; modifications here can significantly alter its ability to engage with different effectors. The functional properties conferred by this domain are central both physiologically—enabling clearance and destruction of pathogens—and therapeutically—as many monoclonal antibodies are engineered specifically for optimized engagement through their **IgG-Fc** domains. Therapeutic manipulation often involves engineering this domain for increased affinity toward activating or inhibitory receptors or modifying glycan structures for improved clinical outcomes.[1][2][3]

Other names

Fragment crystallizable regionFc fragmentIgG FcImmunoglobulin G Fc region

Engagement with immune cell surface receptors (FcγRs) to trigger phagocytosis or cytotoxicity[1][3] Activation of the classical complement pathway via C1q binding[3] Modulation of serum half-life through interaction with neonatal Fc receptor (FcRn)[3]