Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

Hemophilia is an inherited disorder. It prevents blood from clotting properly. People with hemophilia bleed longer than usual. This bleeding can range from mild to severe. In severe cases, people with hemophilia can bleed to death.

Because of the way in which hemophilia is inherited, it almost exclusively affects men. Women can get it, but it is very rare.

Clotting is the process in which the blood thickens or congeals. This prevents a cut or other injury from bleeding endlessly.

Clotting factors are substances in the blood that help the blood to clot. People with hemophilia do not have enough clotting factors.

Blood clotting involves two sets of factors. One set works with specialized blood cells called platelets. Platelets play a central role in blood clotting. The other set of factors involves the coagulation system. Coagulation is a coordinated series of chemical reactions that clots and stops the bleeding.

The main clotting factors involved in hemophilia are Factor VIII, Factor IX and Factor XI.

Types of hemophilia:

  • Hemophilia A is the most common type of hemophilia. People with hemophilia A do not have enough clotting factor VIII. Most people with hemophilia A have severe disease.

  • Hemophilia B is also known as Christmas disease. It is caused by a deficiency in clotting factor IX. It can be mild, moderate or severe.

  • Hemophilia C is also called factor XI deficiency. It is uncommon in the United States. Hemophilia C is caused by a deficiency in clotting factor XI. It is inherited differently than Hemophilia A or B. As a result, it can be passed to both male and female children.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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