RT Book, Section A1 Lichtman, Marshall A. A1 Kaushansky, Kenneth A1 Prchal, Josef T. A1 Levi, Marcel M. A1 Burns, Linda J. A1 Armitage, James O. SR Print(0) ID 1133365995 T1 Hemophilia A and B T2 Williams Manual of Hematology, 9e YR 2017 FD 2017 PB McGraw-Hill Education PP New York, NY SN 9781259642470 LK hemonc.mhmedical.com/content.aspx?aid=1133365995 RD 2024/10/13 AB Hemophilia A and hemophilia B are caused by inherited deficiencies of factor VIII and factor IX, respectively.Both result from decreased production of the deficient factor, production of a factor with decreased functional activity, or a combination of these two abnormalities.The activated form of factor IX, factor IXa, is a serine protease that functions to activate factor X.Activated factor VIII, factor VIIIa, serves as a cofactor, forming a complex with factor IXa on the platelet surface, that dramatically accelerates the rate of factor X activation by factor IXa.In patients with hemophilia, clot formation is delayed because thrombin generation is markedly decreased. The clot that does form is hemostatically ineffective, leading to excessive bleeding.Because deficiency of either factor VIII or factor IX causes an inability to activate factor X, the clinical characteristics and approach to treatment of hemophilia A and hemophilia B are similar.Both hemophilia A and B are X-linked recessive disorders, affecting only males, with rare exceptions (Figure 78–1). Approximately 30% of mutations arise de novo.Hemophilia is found worldwide in all ethnic groups. Hemophilia A is estimated to occur in 1 of 10,000 male births and hemophilia B in 1 of 25,000 to 30,000 male births.