J chain

A J chain is a protein component of the antibodies IgM and IgA. It is a 137 residue polypeptide, encoded by the "IGJ" gene.
Structure.
The J Chain's molecular weight is approximately 15 kDa. It exhibits a standard immunoglobulin folding structure of two β-pleated sheets of four ribbons folded against one another. It has 8 cystine residues. Two of these residues link the α chains of IgA or the μ chains of IgM via disulfide bridges, effectively serving as the "glue" between two Fc regions of the antibody.
The J-chain shows a large degree of homology between avian and human species, suggesting that it serves an important function.
Function.
The J Chain is required for IgM or IgA to be secreted into mucosa. As part of a polymeric immunoglobulin (pIg), the J-chain is essential for binding of pIg to the pIgR, which forms the secretory component upon excretion of the secretory pIg by epithelial cells. This binding facilitates transport of the J-chain positive pIg molecules from the basal to the apical sides of epithelial cells.
Because IgM and IgA are the only two types of antibody that polymerize, initial hypotheses stated 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, however, both of these exist to lesser numbers in organisms lacking J chains. In such cases, there are also fewer IgA dimers. 
The J-chain also plays a role in the activation of complement. J-chain negative IgM hexamers are 15-20 times more effective at activating complement than J-chain positive IgM pentamers. A consequence of this lack of complement activation is it allows J-chain positive pIgM to bind antigens without causing excessive damage to epithelial membranes through complement activation.