An inhibitor is a type of antibody that prevents factor replacement treatment from working. When an inhibitor develops, it binds to factor concentrates such as factor VIII or factor IX, stopping them from working and making bleeding more difficult to treat.
Of people with severe haemophilia A, about 30% will develop inhibitors; among those with mild and moderate haemophilia A the figure is about 9%. However, only 3% of people with haemophilia B will develop an inhibitor.
Known risk factors in the development of inhibitors include:
- severity of the bleeding disorder
- family history of inhibitors
- number of days treated with clotting factor concentrate
Inhibitors typically develop in the first 50 treatment days, with most developing in the first 20 days. This is why when people with severe haemophilia develop inhibitors it is usually during childhood, whilst people with mild or moderate conditions tend to develop them later in life.
Inhibitors are treated with immune tolerance therapy (ITT). ITT involves regular exposure to factor treatment with the aim of the body becoming accustomed to the clotting factor. Studies show that by flooding the system with factor VIII or factor IX the body often stops rejecting it.
It is recommended that children with inhibitors start ITT therapy as early as possible, and usually continue without interruption for 12 months or longer.