Bound secretory component immunoglobulin

Besides L and H chains, secretory IgA contains a polypeptide called secretory component, or SC, which has a molecular weight of 65,000 — 70,000 and is bound to the H chains of the molecule. Another component, J chain (J for joining mol. wt. 15,000), is found in the IgM of many primitive and advanced species, and in polymeric, but not monomeric (four-chain) IgA. J chain is linked to H chains through disulfide bonds. The amino acid sequences and antigenic properties of SC and J chain do not appear to be homologous to those of the immunoglobulin polypeptides i.e., they do not seem to have a common evolutionary origin. 

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