Isolation and Characterization of Intermediate Complexes and Other Components with Common Antigenic Determinants

Fractions with affinity for heat‐denatured human IgG (HDG) were isolated from four sera that contained intermediate complexes (IC). These IC fractions contained part of the 7S IgG, all IC, and part of the rapidly sedimenting complexes (RC) found in the sera. The IC consisted of IgG1 or IgG3 and the RC of IgG and IgM with kappa and lambda light chains. The IgG in the IC fractions contained an abnormally large amount of neuraminic acid. No correlation between IgG subclass or content of neuraminic acid and complex formation was found. There were indications that the formation of IC was not only the result of a self‐association of IgG molecules with anti‐γ‐globulin activity. Specific rabbit antisera were prepared against two of the IC fractions. Affinity chromatography with immobilized IgG and F(ab')2γ from these antisera confirmed the presence of common antigenic determinants in the sera. These determinants occurred mainly in 7S components in one individual, in IC in one and in RC in another. Only a minor part of the serum components with affinity for HDG contained the determinants. RF activity was found in the components that lacked and in those that contained the common antigenic determinants.