People with haemophilia receive factor concentrate to help their blood to clot. However, in some people, the immune system will then start to produce antibodies to block the effects of the treatment. These antibodies are known as ‘inhibitors’.
The inhibitor can make the medication used to treat haemophilia less effective. This means bleeds become more difficult to control. This can lead to more bleeding into muscles and joints resulting in more joint damage and pain. When this happens, more intensive or alternative treatment may be required to control bleeds or to attempt to eliminate inhibitors.
Most inhibitors are temporary. They most often appear during the first year of treatment but they can appear at any time.
Some people with severe haemophilia develop an antibody or inhibitor to factor treatment. The effect of this is that when the factor is given for a bleed there is no clinical response because the activity is immediately neutralised. Often the first sign that a person has inhibitors is that his treatment does not appear to be working.
Inhibitors of factor VIII occur in about twenty percent of individuals with factor VIII deficiency. The inhibitors can be high titre or low titre. High titre inhibitors refer to a higher level of inhibitors which is more clinically significant and more difficult to eradicate. Low titre inhibitors are often transient and the person can still be treated with factor concentrates at higher doses. If a child develops a high titre factor VIII inhibitor, the optimum therapy is immune tolerance therapy where the child is treated with very high doses of factor VIII for an average period of about one year, in an attempt to eradicate the inhibitors. Treatment for factor VIII inhibitors can be with a number of agents including a recombinant factor VIIa (Novo Seven) or an alternative product known as FEIBA.
Inhibitors in patients with factor IX deficiency are extremely rare and are more difficult to treat. In general, children and adults with haemophilia are screened for inhibitors at least once or twice per annum.