Inhibition of IgG autoantibody activity by IVIgM. Increasing amounts of IVIgM (▵), IVIgM depleted of its rheumatoid factor activity (○), IVIg (□), a human monoclonal IgM rheumatoid factor (•), or monoclonal IgM (▴) were coincubated with a fixed amount of ¹²⁵I-labeled affinity-purified anti-TG or anti-DNA IgG from 2 patients with Hashimoto's thyroiditis and 2 patients with SLE overnight at 4°C. The binding of ¹²⁵I-IgG to the corresponding antigen was then assessed. (Left panels) inhibition of anti-TG activity; (right panels) inhibition of anti-DNA activity. The abscissa indicates molar ratios between competitor Ig and ¹²⁵I-IgG autoantibody.