Your 20s
& Beyond
What are Inhibitors?
The Basics
Patients born with haemophilia A or B (hereditary haemophilia) usually receive treatment known as clotting factor replacement therapy, which helps their blood to clot by replacing the missing Factor VIII or Factor IX respectively. Some patients, however, develop antibodies (inhibitors), which can be a serious complication because they prevent this factor replacement therapy from working. As a result of this, bleeding episodes in a person with haemophilia and an inhibitor are more difficult to treat.
Inhibitors are less common in those with haemophilia B and those with mild or moderate haemophilia A, occurring most often in children with severe haemophilia A. They can appear after only a few treatments and are classified by their level of antibody response to their injection of factor. If their body produces large quantities of antibodies in response to their injection of factor, they are known as a high responder, whereas if the level of antibody does not increase much after an injection of factor they are known as a low responder.
In addition, people with acquired haemophilia also have inhibitors to their clotting factor. In this case their immune system believes the body's own clotting factor is foreign and makes antibodies (inhibitors), which neutralise it (click here for more information).
Advanced
Inhibitors are antibodies to a clotting factor.
People who have developed inhibitors are classed as either high-or low-responders.
The expression 'high responder' or 'low responder' refers to the way an individual's inhibitor reacts to an infusion of clotting factor.
The strength of an inhibitor (called inhibitor titre) is measured by the Bethesda Assay and reported in Bethesda Units, so people may have a high or low titre inhibitor.
How common are inhibitors?
It is estimated that up to one third of people with severe or moderate haemophilia A develop them. However, less than 5% of people with severe or moderate haemophilia B develop inhibitors.
Development of inhibitors is rare in people with mild haemophilia.
Inhibitors tend to develop more frequently during childhood, usually within the first 50 infusions of factor replacement therapy and when children's immune systems are developing.
