Immunoglobulin M and secretory immunoglobulin A: presence of a common polypeptide chain different from light chains.

Unique polypeptide chains have been isolated from S-sulfonated light-chain fractions of human serum immunoglobulin M and colostral immunoglobulin A. Their electrophoretic mobilities, molecular weights, peptide maps, amino acid compositions, and antigenic determinants are very similar or perhaps identical but differ from those of light chains and secretory piece.