A 15 kD "joining" peptide that forms one of the linkages between monomers of immunoglobulin a or immunoglobulin m in the formation of polymeric immunoglobulins. There is one J chain per one IgA dimer or one IgM pentamer. It is also involved in binding the polymeric immunoglobulins to POLYMERIC IMMUNOGLOBULIN RECEPTOR which is necessary for their transcytosis to the lumen. It is distinguished from the immunoglobulin joining region which is part of the immunoglobulin variable region of the immunoglobulin light and heavy chains.