Mechanism of action of neonatal Fc receptors in maintaining IgG levels. (A) Protection of IgG from degradation, and (B) how FcRn inhibitors disrupt IgG recycling. In (A), IgG is ingested by pinocytosis. Pinocytotic vesicles fuse with acidic endosomes in which FcRn can bind IgG. Excess unbound IgG and other proteins enter the lysosome and are degraded. IgG bound to FcRn is retained and released by exocytosis. In (B), FcRn inhibitors bind to FcRn in both neutral and acidic environments. In the presence of FcRn inhibitors, ingested IgG is unable to bind to FcRn; the unbound IgG enters the lysosome and is degraded. For illustrative purposes, albumin binding is not shown. Reproduced from Patel and Bussel ³⁷ with permission under CC license.