Hemophilia occurs when one of the blood-clotting proteins known as "factors" is missing -- or the amount present is too low -- for normal clotting to occur. Factor VIII is missing or too low in about 80 percent of hemophilia cases. Factor IX is deficient in about 20 percent of cases.

The process of blood clotting starts the moment damage to the blood vessel wall occurs. Platelets -- small, sticky discs that float in the blood -- begin collecting at the damaged site. As more platelets collect, a plug forms in the hole to stop bleeding. Although the platelet plug can hold for a while, without a "glue" to permanently hold the platelets together, the plug breaks apart and bleeding begins again. The "glue" that holds the plug together is called fibrin. Factor VIII, Factor IX, and other blood-clotting proteins are required for the production of fibrin.

The body produces the different clotting factors based on the instructions from a gene. In hemophilia, the gene that causes factor to be produced is defective. In most cases of hemophilia, the defective gene is passed from parent to child. In cases where there is no family history of the disorder, hemophilia occurs due to a change or mutation of the gene. In such cases, the mutation may only be discovered after the birth of a son with hemophilia.

First published February 1, 2000
Last updated December 2, 2002
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

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