ABSTRACT
A 17.6-kDa polypeptide chain present in polymeric immunoglobulins, including both IgM and IgA. It links fourchain immunoglobulin monomers to produce the polymeric immunoglobulin structure. J chains are produced in plasma cells and are incorporated into IgM or IgA molecules prior to their secretion. Incorporation of the J chain appears essential for transcytosis of these immunoglobulin molecules to external secretions. The J chain comprises 2 to 4% of an IgM pentamer or a secretory IgA dimer. Tryptophan is absent from both mouse and human J chains. J chains are composed of 137-amino-acid residues and a single, complex,
N
-linked oligosaccharide on asparagine. Human J chain contains three forms of the oligosaccharide which differ in sialic acid content. The J chain is fastened through disulfide bonds to penultimate cysteine residues of
µ
or
α
heavy chains. The human J chain gene is located on chromosome 4q21, whereas the mouse J chain gene is located on chromosome 5.
