Antibody differentiation: Apparent sequence identity between variable regions shared by IgA and IgG immunoglobulins

We have analyzed a pair of human myeloma immunoglobulins (biclonal proteins) of the IgG and IgA classes from a single patient, GR. The light chains are identical in amino-acid sequence over 40 residues at their NH2-terminus, whereas the heavy chains are identical throughout 45 residues of their NH2-terminus. Additional chemical and serological studies suggest the light chains and variable regions of the heavy chain (VH) are very similar, if not identical. The implications of these and of other published studies are discussed with regard to (i) the association of one VH region with multiple constant regions of the heavy chain (CH regions), (ii) two alternative types of V-C joining mechanisms, (iii) the differentiation of antibody-producing cells, and (iv) three categories of biclonal immunoglobulins.