High concentrations of apparently monoclonal IgA and IgM were found in the serum of a single individual (SC). Both proteins had κ light chains of Inv-3 allotype. Rabbit antisera against the isolated SC IgA or SC IgM revealed that the two proteins share identical individually specific antigenic (ind) determinants, indicating the presence of common antigenic regions in the variable portions of SC IgA and SC IgM in spite of their differences in the class-specific constant portions. The common ind determinants were found on the Fab fragments, but not on the separated heavy or light chains. The ind determinants were restored when the separated component chains were recombined.
This work was supported in part by Grant AI-8472 from the National Institute of Allergy and Infectious Diseases.