Inhibitors are antibodies to factor VIII or factor IX made by the body’s immune system that attack and destroy the factor VIII and IX proteins in clotting factor concentrates, making treatment ineffective.

They appear almost exclusively in patients with severe hemophilia. There is some controversy over the precise incidence (number of new cases) of inhibitor development, but it is generally accepted that between 10 and 30 percent of people with severe haemophilia A will develop inhibitors at some stage. By contrast, inhibitor development in haemophilia B is very rare indeed, and seen in 1 to 3 percent of subjects.

Most inhibitors emerge after relatively few treatments. In general, the more treatments a person has had without developing inhibitors, the less likely he is to develop an inhibitor.

Treatments exist that can sometimes eliminate inhibitors. In other cases, they disappear naturally. In other cases, they continue for many years.