Characterization of an antibody to factor VIII in a patient with acquired hemophilia with circulating immune complexes.

A 73-year-old previously healthy woman was admitted because of severe bleeding from esophagitic lesions and intraabdominal bleeding following hysterectomy. Acquired hemophilia, probably due to an IgG antibody to factor VIII (64 inhibitor units/ml) was noticed, the VIII:C in the patient's plasma being 18% or normal. Immune complexes isolated by polyethylene glycol precipitation had only a weak factor VIII inhibiting activity whereas IgG purified from the complexes and monomeric IgG present in her plasma exerted a strong inhibition. Removal of the complexes from plasma had no effect on the inhibitor titer thus indicating that only a minor part of the antibody was circulating as immune complexes. Plasma or purified IgG from the patient decreased the VIII:C of normal plasma to 18 og 14%, respectively, total inhibition being impossible to achieve even in antibody excess, probably reflecting residual activity of factor VIII bound to the patient's antibodies. The ristocetin cofactor activity of normal plasma was unaffected by the antibodies. Transfusion of factor VIII concentrate to the patient resulted in therapeutic levels of circulating factor VIII and transfused factor VIII circulated longer than usual. Partial remission of the disease with adequate levels of VIII:C occurred after 3 months.