Immunochemical evidence for a common variable region in three immunoglobulin classes in the same individual.

One IgG1(kappa), one IgM(kappa), and one IgA1(kappa) monoclonal (M)-component were purified from one human serum. Rabbit antisera were raised against the IgG and IgM M-components and were absorbed until specific for idiotypic determinants on these molecules. All three M-components gave reactions of immunological identity when tested by double radial immunodiffusion with either of the two idiotype-specific antisera. Both heavy and light chains were isolated from each of the three M-components and all preparations inhibited formation of idiotypic precipitates. None of these preparations formed precipitates with idiotype-specific antisera alone. When heavy or light chains of one M-component were hybridized with light or heavy chains from the other M-components the resultant molecules precipitated with anti-idiotypic serum. Hybrids with chains from polyclonal IgG were not precipitable with such antiserum. These results indicate that the variable region of the heavy chains of these M-components of three different immunoglobulin classes are closely similar, if not identical.