Factor VIII antibody in a patient with mild haemophilia.

We present the rare occurrence of an inhibitor of factor VIII procoagulant arising in a patient with mild haemophilia A and rheumatoid arthritis. The inhibitor was transient and behaved like a low titre, type II factor VIII procoagulant inhibitor similar to previously reported cases (Biggs et al, 1972b). In vitro studies confirmed the type II-like interaction of this inhibitor with the factor VIII procoagulant molecule. Factor VIII procoagulant antigen level was equal to the factor VIII procoagulant activity, which excluded dysproteinaemia as the cause. This patient's HLA type has no known association with abnormal immune responsiveness or autoimmune disease, and his clinical course as well as in vitro studies were similar to the eight previously reported cases of factor VIII procoagulant inhibitors arising in mild haemophilia A.