Hemophilia A is caused by low or absent levels of functioning factor VIII (factor 8). Factor VIII is a protein that is necessary for blood to clot properly. When it is absent, spontaneous bleeding can occur, most often in the joints. When factor VIII levels are present, bleeding can be variable in that some people may have joint bleeding without much trauma, whereas others only have bleeding with major surgery. The diagnosis is made by demonstrating low levels of factor VIII in the blood that is not due to other causes. Sometimes examination of the factor VIII gene is needed to confirm the diagnosis. Hemophilia A is treated by increasing factor VIII levels in the blood either by infusions of factor VIII into the vein or by using desmopressin that can increase circulating factor VIII levels by releasing stored factor VIII. Hemophilia B is similar to hemophilia A except that the missing protein is factor IX. Factor IX is also essential for blood to clot normally. One difference, is that the only way to increase factor IX in the blood stream is the infuse factor IX into the vein.

In addition to joint damaged caused by bleeding, one important complication of hemophilia is the development of an inhibitor. An inhibitor is an antibody that binds factor VIII or IX because it is different than what the body makes. In the same way that an antibody might bind to a bacteria when you have an infection. An inhibitor occurs in about a 25% of patients with severe hemophilia, about 10% of mild hemophilia A and 3% of severe hemophilia B patients.