What causes haemophilia?
In a normal individual haemostasis, the ability to stop bleeding, is a complex and finely regulated process that depends on the interaction of three components: blood vessels, blood platelets and coagulation (clotting) factors. Clotting factors are multiple plasma proteins (numbered I to XIII) that cause cascades of enzymatic reactions in blood plasma. Basically, these cascades of reactions are often divided into an intrinsic, an extrinsic and a common pathway. Patients with haemophilia A have a mutation (an abnormality) of the gene coding for coagulation Factor VIII, leading to abnormally low plasma levels of Factor VIII. This factor is involved in the intrinsic pathway of coagulation. Haemophilia B is the result of mutations of the gene coding for clotting Factor IX. As this factor is also involved in the intrinsic pathway of coagulation, it is not surprising that low levels of Factor IX give rise to a disease identical to haemophilia A.
