The Joining (J) chain is a protein component that links monomers of antibodies IgM and IgA to form polymeric antibodies capable of secretion. The J chain regulates the multimerization of IgM and IgA in mammals. When expressed in cells, it favors the formation of a pentameric IgM and an IgA dimer. The J chain is incorporated late in the formation of IgM polymers and thermodynamically favors the formation of pentamers as opposed to hexamers. The J chain facilitates IgA dimerization by linking two monomer secretory tails. Structurally, the J chain joins two antibody monomers asymmetrically by forming intermolecular disulfide bonds and bringing hydrophobic β-sandwiches on each molecule together.