J chain

IgM (Immunoglobulin M) antibody molecule consisting of 5 base units.
1: Base unit.
2: Heavy chains.
3: Light chains.
4: J chain.
5: Intermolecular disulfide bonds.

The dimeric IgA molecule.
1: H-chain
2: L-chain
3: J-chain
4: secretory component

J chain is a protein component of the antibodies IgM and IgA.

Because IgM and IgA are the only two types of antibody that polymerize, it was initially thought that J chain was required for polymerization. However, it was subsequently found that IgM is able to polymerize in the absence of J chain as both a pentamer and a hexamer.

It is now believed that J chain is necessary to enable secretion of antibodies into mucosal tissues.