Immunoglobulins are transported across a variety of epithelial tissues. The best studied example of this is the transport of polymeric IgA and IgM by the polymeric immunoglobulin receptor (pIgR) across many types of epithelial cells. The pIgR binds its ligand at the basolateral surface and is internalized into endosomes. Here it is sorted into vesicles that transcytose it to the apical surface. At the apical surface the pIgR is proteolytically cleaved, and the large extracellular fragment (known as secretory component) is released together with the ligand. The pIgR contains a cytoplasmic domain of 103 amino acids that contains several sorting signals. Targeting from the trans-Golgi network to the basolateral surface is determined by the membrane-proximal 17 residues of this domain. There are two endocytosis signals that contain crucial tyrosine residues. Transcytosis of the pIgR is stimulated by binding of polymeric IgA. Phosphorylation of a cytoplasmic serine promotes transcytosis of the pIgR without ligand bound. Transcytosis may be regulated by the heterotrimeric Gs protein, protein kinase C and calmodulin. IgG is transcytosed from the apical to basolateral surface in several epithelial tissues such as the placenta and the small intestine of newborn rats. The receptor for intestinal transport of IgG is structurally similar to class I MHC molecules.