May also be called: Hemophilia A

Hemophilia is a disease that prevents blood from clotting properly. A clot helps stop bleeding after a cut or injury.

In factor VIII deficiency (hemophilia A), the body doesn't make enough factor VIII (factor 8), one of the substances the body needs to form a clot.

More to Know

When most people get a cut, the body naturally protects itself. Sticky cells in the blood called platelets go to where the bleeding is and plug the hole. This is the first step in the clotting process. When the platelets plug the hole, they release chemicals that attract more sticky platelets and also activate various proteins in the blood known as clotting factors. These proteins mix with the platelets to form fibers, and these fibers make the clot stronger and stop the bleeding.

Our bodies have 12 clotting factors that work together in this process (numbered using Roman numerals from I through XII). Having too little of factors VIII (8) or IX (9) is what causes hemophilia. A person with hemophilia will lack only one factor, either factor VIII or factor IX, but not both. There are two major kinds of hemophilia: hemophilia A, which is a factor VIII deficiency; and hemophilia B, which is a factor IX deficiency.

People with hemophilia may bruise and bleed easily, and they may bleed a lot or for a long time after an injury. Bleeding can occur anywhere in the body, such as into the joints, muscles, or digestive tract. Some people have mild disease, some moderate, and some more severe.

Hemophilia is a genetic disorder, which means it's the result of a change in genes that was either inherited (passed on from parent to child) or occurred during development in the womb. Hemophilia almost always occurs in males, but in rare cases can affect females.

Keep in Mind

Doctors diagnose hemophilia by performing blood tests. Although the disease can't be cured (except by a liver transplant — which sometimes can cause health problems worse than hemophilia itself), it can be managed.

Patients with more serious cases of hemophilia often get regular shots of the factor that they're missing — known as clotting factor replacement therapy — to prevent bleeding episodes. The clotting factors are transfused through an intravenous (IV) line, and can be given in the hospital, at the doctor's office, or at home.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.