The joining (J) chain is a small polypeptide, expressed by mucosal and glandular plasma cells, which regulates polymer formation of immunoglobulin (Ig)A and IgM. J-chain incorporation into polymeric IgA (pIgA, mainly dimers) and pentameric IgM endows these antibodies with several salient features. First, a high valency of antigen-binding sites, which makes them suitable for agglutinating bacteria and viruses; little or no complement-activating potential, which allows them to operate in a noninflammatory fashion; and, most importantly, only J-chain-containing polymers show high affinity for the polymeric Ig receptor (pIgR), also known as transmembrane secretory component (SC). This epithelial glycoprotein mediates active external transfer of pIgA and pentameric IgM to exocrine secretions. Thus, secretory IgA (SIgA) and SIgM, as well as free SC, are generated by endoproteolytic cleavage of the pIgR extracellular domain. The secretory antibodies form the 'first line' of defence against pathogens and noxious substances that favour the mucosae as their portal of entry. The J chain is involved in creating the binding site for pIgR/SC in the Ig polymers, not only by determining the polymeric quaternary structure but apparently also by interacting directly with the receptor protein. Therefore, both the J chain and the pIgR/SC are key proteins in secretory immunity.