Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

Figure 4

Transcytosis of pIgR through a polarized epithelial cell and formation of secretory Igs. A polarized columnar epithelial cell is illustrated, with the apical surface at the top and the basolateral surface at the bottom. Newly synthesized pIgR is targeted to the basolateral surface, where it binds polymeric Ig (pIg; illustrated here as dimeric (d)IgA) with or without bound antigen (Ag). Following receptor-mediated endocytosis, pIg-bound and unoccupied pIgR molecules are transported through a series of intracellular vesicles to the apical surface. Proteolytic cleavage of pIgR at the extracellular face of the plasma membrane releases free secretory component (SC) and secretory SIg (illustrated here as SIgA). Modified from [13] with permission from John Wiley and sons.