Figure 1.

FcRn mediates bidirectional transport and membrane recycling of IgG in epithelial cells. IgG is believed to enter the cell by fluid phase pintocytosis (assuming neutral pH condition) and does not bind to FcRn until the endosome is acidified. However, in the duodenum, the acidic luminal environment may allow IgG to bind to FcRn at the apical membrane surface before endocytosis.⁷⁹ Two FcRn bind to a single IgG molecule. IgG may be transcytosed to the opposite membrane surface or recycled back to the same membrane surface.¹¹ The exact intracellular pathway is believed to differ between cells. IgG is dissociated from FcRn at the membrane surface at neutral pH.