Immunoglobulin J chain is a 137-residue polypeptide that is covalently linked to polymeric immunoglobulins and participates in their synthesis and transport to external secretions [PMID: 3929246]. It mediates the polymerization of both IgM and IgA immunoglobulins. Its synthesis is closely regulated in B lymphocytes, apparently at the level of RNA transcription [PMID: 6278047].