Abstract
The J chains isolated from polymeric IgA and IgM were shown to be distinct from the other immunoglobulin subunits, including the secretory component, both on the basis of amino-acid content and antigenic determinants. By the same criteria, the J chains were found to be indistinguishable from each other whether they were derived from pentamer IgM, polydisperse myeloma IgA, or dimer colostral IgA. The only differences were observed in (a) the extent of disulfide bond cleavage required for J chain release from the different polymers, and (b) the three-dimensional arrangement of J chains in the different polymers. These data support the hypothesis that the same J chain joins the monomeric units of IgA and IgM to form their respective polymeric molecules.