What causes haemophilia?

People with haemophilia lack an essential protein in their blood called a ‘clotting factor’. There are normally 12 clotting factors circulating in the blood at any one time, and all play an important part in the rather complex process that helps blood to clot. If any one of these factors is missing or reduced sufficiently, blood clotting can be affected, leaving an individual at risk of internal or external bleeding.

• People with haemophilia A are deficient in the essential clotting factor, Factor 8 (usually written in roman numerals – Factor VIII)
• People with haemophilia B are deficient in the essential clotting factor, Factor 9 (Factor IX).

The reason people with haemophilia don’t have these clotting factors is usually that they have inherited a ‘haemophilia’ gene from their parents. Haemophilia is a genetic disorder, which is usually inherited. It cannot be caught or transmitted except through inheritance. The ‘haemophilia’ gene is passed down from a parent to a child.

However, about one third of new cases are caused by a new mutation of the gene in the mother or the child. In these cases, there is no previous history of haemophilia in the family.