Inhibitors are a serious medical problem that can occur when a person with hemophilia has an immune response to treatment with clotting factor concentrates.

The immune system defends the body from harmful germs and viruses. Sometimes in the case of an inhibitor, a person’s immune system reacts to proteins in factor concentrates as if they were harmful foreign substances because the body has never seen them before. When this happens, inhibitors (also called antibodies) form in the blood to fight against the foreign factor proteins. This stops the factor concentrates on being able to fix the bleeding problem.

Bleeding is very hard to control in someone with hemophilia who develops inhibitors. A person with inhibitors faces more bleeding and pain because treatment with factor concentrates does not work. In patients with persistent inhibitors, if bleeding into the muscles and joints (the most common type of bleeding in hemophilia) is not controlled, the permanent joint damage is likely.

For patients with sufficient access to care, treatment of inhibitors is one of the biggest challenges in hemophilia today. It is possible to get rid of inhibitors using a technique called immune tolerance induction. However, this type of treatment requires specialized medical expertise, is expensive, and takes a long time.

Drugs called bypassing agents can be used to work around inhibitors and help blood clot.

Content developed by the WFH Inhibitors Working Group