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Hemophilia type A is an inherited bleeding disorder that is caused by a deficiency of factor VIII (a blood protein required for blood clot formation). An acquired form of hemophilia can develop in nonhemophiliacs when the immune system begins making autoantibodies that inhibit the activity of a small portion of the factor VIII molecule called factor VIII:C. This antibody is known as factor VIII:C inhibitor. Acquired hemophilia has been treated by giving intravenous injections of purified factor VIII:C. The clinical effectiveness of this form of treatment is related to the amount of factor VIII:C inhibitor present in the blood. Factor VIII:C purified from pig (porcine) blood has been used because it is less likely to react with the factor VIII:C inhibitors present in the patient's blood than factor VIII:C purified from human blood. This means that less of the porcine factor VIII:C will interact with the inhibitors (autoantibodies) and more will be available to prevent bleeding. Transfusion reactions are rare following treatment with porcine factor VIII:C. Adverse reactions of any type to porcine factor VIII:C occur in 7 to 13 percent of patients, and most of these reactions are mild or transient. Severe reactions are uncommon. Using porcine factor VIII:C instead of human factor VIII:C reduces the transmission of AIDS and hepatitis. (Consumer Summary produced by Reliance Medical Information, Inc.)