Immunochemical characterization of factor VIII inhibitors.

Factor VIII inhibitors are antibodies, predominantly of the IgG class, that inactivate factor VIII procoagulant activity. They do not fix complement and cannot be detected by standard immunoprecipitin assays. Their immunochemical properties are unusual, for the alloantibodies have consistent light chain restriction (kappa) and most allo- and auto- anti-VIII:C are predominantly or exclusively IgG4. This property may be responsible for the lack of complement fixation and immunoprecipitation. Human anti-VIII:C react with several different VIII:C antigenic determinants, and recent studies have begun to characterize the major epitopes. In addition, anti-VIII:C idiotypes have been identified using both rabbit anti-idiotype serum and monoclonal antibodies. These reagents provide a new way to obtain more precise immunochemical characterization.