What Is Hemophilia?

Hemophilia (heem-o-FILL-ee-ah) is a rare, inherited bleeding disorder in which your blood doesn’t clot normally. If you have hemophilia, you may bleed for a longer time than others after an injury. You also may bleed internally, especially in your knees, ankles, and elbows. This bleeding can damage your organs or tissues and, sometimes, be fatal.

People born with hemophilia have little to none of a protein needed for normal blood clotting. The protein is called a clotting factor. There are several types of clotting factors, and they work together with platelets to help the blood clot. Platelets are small pieces of blood cells that are formed in the bone marrow. They play a major role in blood clotting. 

When blood vessels are injured, clotting factors help the platelets stick together to plug cuts and breaks at the site of the injury to stop the bleeding. Without clotting factors, normal blood clotting can’t take place. Sometimes people with hemophilia need injections of a clotting factor or factors to stop bleeding.

There are two main types of hemophilia. If you have hemophilia A, you have little to no clotting factor VIII (8). About 9 out of 10 people with hemophilia have type A. If you have hemophilia B, you’re missing or have low levels of clotting factor IX (9). 

Hemophilia can be mild, moderate, or severe, depending on how much clotting factor is in the blood. About 7 out of 10 people who have hemophilia A have the severe form of the disorder. People who don’t have hemophilia have a factor VIII activity of 100 percent; people who have severe hemophilia A have a factor VIII activity of less than 1 percent. 

In addition to being inherited, hemophilia also can be acquired, which means that you can develop it during your lifetime. It can develop if your body forms antibodies to the clotting factors in your bloodstream. The antibodies can block the clotting factors from working. Only inherited hemophilia is discussed in this article.

About 18,000 people in the United States have hemophilia. Each year, about 
400 babies are born with the disorder. Hemophilia usually occurs only in males (with very rare exceptions).