Immunoglobulin J chain, also known as IGJ and IGCJ, is a secreted polypeptide which is the first immunoglobulin-related polypeptide expressed during the embryogenesis and differentiation of B cells in the fetal liver. The joining Immunoglobulin J chain is a small polypeptide, expressed by mucosal and glandular plasma cells, which regulates polymer formation of immunoglobulin (Ig)A and IgM. Immunoglobulin J chain / IGJ serves to link two monomer units of either IgM or IgA. In the case of IgM, the J chain-joined dimer is a nucleating unit for the IgM pentamer, and in the case of IgA, it induces larger polymers. Immunoglobulin J chain / IGJ also helps to bind these immunoglobulins to the secretory component. J-chain incorporation into polymeric IgA (pIgA, mainly dimers) and pentameric IgM endows these antibodies with several salient features. Immunoglobulin J chain / IGJ is involved in creating the binding site for pIgR / SC in the Ig polymers, not only by determining the polymeric quaternary structure but also by interacting directly with the receptor protein. Both the immunoglobulin J chain / IGJ and the pIgR/SC are key proteins in secretory immunity.