Some patients develop an immune response to the medications used to treat hemophilia. The immune system is the primary defense system of the body against disease and foreign agents. For these patients, the immune system produces antibodies that “inhibit” clot formation by destroying the clotting factor before it has a chance to stop the bleeding. The reason for a patient developing inhibitors is still not entirely clear and the presence of an inhibitor makes treating bleeds more difficult. Standard treatment does not work, and other methods to control bleeding must be used.