Transepithelial transport of IgA and IgM across the mammary epithelial cells occurs via the polymeric immunoglobulin receptor (pIgR) which is responsible for binding dimeric IgA and pentameric IgM in mucosal tissues [149, 150] . The polymeric nature of IgA and IgM arises from their binding with the J-chain peptide [116] . Only IgA or IgM that contain the J chain have a high affinity for pIgR [116, 151, 152] . In fact, the J chain has been evolutionarily conserved within tetrapods to the point where human polymeric IgA can bind to the pIgR from the amphibian Xenopus laevis [152] . Polymeric IgA or IgM bound to pIgR is internalized and transported to the apical end of the mammary epithelial cell by an endocytic process. The pIgR molecule is cleaved to release a receptor fragment, called secretory component (SC), which remains bound to the immunoglobulin molecule [119, 149] . In the case of pIgR receptor sites that are not occupied by immunoglobulin, the secretory component is still cleaved from the membrane-bound portion of pIgR, resulting in release of free secretory component. The secretory component has protective effects of its own, potentially blocking epithelial adhesion of enterotoxigenic E. coli and neutralizing the effects of other pathogens [148] .