Studies of complement activation and IgG subclass restriction of anti-thyroglobulin

Using a sensitive radioimmunoassay, it was demonstrated that autoantibodies to human thyroglobulin (hTg) did not activate complement upon interaction with solid phase adsorbed hTg. Since IgG4 immunoglobulins do not activate complement, we studied sera containing large amounts of anti-thyroglobulin (anti-Tg) with a sensitive radioimmunoassay for IgG4 anti-Tg. Antibodies of the IgG4 subclass were detected in the sera of each of the eight patients studied and not in control subjects. Quantitative assays of IgG4 anti-Tg were performed by comparing the removal of anti-Tg and total IgG4 during immunoadsorption of anti-Tg. The contribution of IgG4 anti-Tg antibodies to the total quantity of anti-Tg ranged from 6 to 30%, an amount inadequate to explain the lack of complement activation by anti-Tg.