is a rare form of hemophilia in which the body makes specialized proteins called autoantibodies that attack and disable coagulation factors (proteins in the blood that assist with the clotting process). The most common coagulation factor attacked is coagulation factor VIII, though other coagulation factors, including factor V and factor IX have been associated with the condition. The production of autoantibodies is sometimes associated with pregnancy, immune system disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis, Sjogren syndrome), viral infections cancer, or allergic reactions to certain drugs. In about half of cases, the cause of acquired hemophilia is unknown. Treatment may vary from person to person. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.