Antibodies to Factor VIII. I. Variations in Stability of Antigen-Antibody Complexes in Hemophilia A

Human factor VIII-anti-factor VIII complexes were formed in vitro in slight antigen excess, using plasma of hemophiliacs who were found to have antibodies neutralizing AHF activity. These complexes, stable at +37°C and pH 7.4, were submitted to classical procedures known to favor dissociation of antibody from antigen. The methods used to obtain dissociation, incubations at +56°C and at pH 4.2, inactivated both unbound factor VIII and that released as a consequence of dissociation. The extent of dissociation was measured by the recovery of anti-factor VIII activity. Increasing resistance of complexes towards dissociation was observed in the plasma of the patients whose titer of inhibitor was increasing after recent transfusions. These observations suggested the emergence, as a direct consequence of renewed antigenic stimulation, of a population of different antibodies characterized by higher combining strength.