Additional polypeptides in polymeric forms of IgA and IgM

The polymeric forms of IgA and IgM contain another polypeptide, the J-chain, which has no homology with other Ig components but is important in the joining of the four-chain monomers. IgM of 

Irrespective of the number of monomers, only one J-chain is found per polymer. In the case of IgM, the J-chain-containing dimer seems to serve as a nucleating unit to promote S-S binding among the other IgM monomers (Garcia Pardo et al., 1977). The plasma cell synthesizes both antibody and J-chain. 

SC has at least two important functions (i) it transports the secretory IgA (sIgA) through the epithelium to the lumen, and (ii) it confers a greater resistance of the sIgA in the mucous secretions or colostrum against proteolytic attack. Sometimes, IgM with SC is found in patients with an IgA deficiency. 

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