Existence of disulfide-bonded IgM-IgA, IgM-IgG and IgM-IgG fragment complexes in one patient

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Abstract
The existence of disulfide‐bonded immunoglobulin (Ig) complexes of IgM‐IgA, IgM‐IgG, and IgM‐IgG, in addition to a monoclonal IgM (x) paraprotein, free kappa chains and fragments of IgG (FIgG), were found in the plasma of a multiple gammopathy patient. This is the first report on the interclass disulfide‐bonded Ig complexes. Upon exposure to a dissociating buffer containing 6 M urea, 0.1 M Tris‐HCl, pH 8.0, a small portion of the IgM‐IgA complexes and most of the IgM‐IgG as well as the IgM‐FIgG complexes dissociated, but the majority of the IgM‐IgA and small amounts of other complexes remained intact. These intact complexes were not the result of antigen‐antibody reactions and were apparently held together by disulfide bonds which could be broken into monomeric Ig basic units (H‐L)2 and smaller components by mild reduction and alkylation.