Why do some patients develop inhibitors?
Our body’s immune system is the primary defense against viral diseases and other foreign agents. Some patients with bleeding disorders can develop an immune response against the very medications used to treat their disorder. For these patients, their immune system produces antibodies that “inhibit” clot formation by destroying clotting factor before it has a chance to work on stopping a bleed. The precise reason why a patient develops an inhibitor is still not entirely clear. Most researchers agree though that it can be attributed to a combination of factors – a genetic predisposition, and/or other environmental factors (such as type of product, age at first treatment, treatment intensity, surgery or infections).
Yet the presence of an inhibitor clearly makes treating bleeds more difficult. Standard treatment does not work, and other methods or products need to be used to control bleeding.
Some patients develop an immune response to the medications used to treat hemophilia. Experts estimate that 1/3 to 1/5 of people with severe hemophilia A may develop an inhibitor sometime in their lives. For people with severe hemophilia B, inhibitors are somewhat less common, thought to affect between 1% to 4% of this group.
An inhibitor is more likely to develop in early childhood, and especially during the first 50 treatment exposure days. The incidence of inhibitors is highest among those with severe hemophilia, followed by moderate and mild deficiency or moderately severe hemophilia. The risk of inhibitor development is higher if someone in the patient's family also has an inhibitor, and some studies have shown it may be more prevalent among African Americans. While rare, there are also patients with von Willebrand disease who develop inhibitors.