Hemophilia is a rare bleeding disorder in which the 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 inside your body (internally), especially in your knees, ankles and elbows. This bleeding can damage your organs and tissues and may be life threatening.
Hemophilia usually is inherited. "Inherited” means that the disorder is passed from parents to children through genes.
People born with hemophilia have little or no clotting factor. Clotting factor is a protein needed for normal blood clotting. There are several types of clotting factors. These proteins work with platelets to help the blood clot.
Platelets are small blood cell fragments that form in the bone marrow — a sponge-like tissue in the bones. Platelets play a major role in blood clotting. When blood vessels are injured, clotting factors help platelets stick together to plug cuts and breaks on the vessels and stop bleeding.
The two main types of hemophilia are A and B. If you have hemophilia A, you're missing or have low levels of clotting factor VIII (8). About 80% of people who have hemophilia have type A. If you have hemophilia B, you're missing or have low levels of clotting factor IX (9).
Rarely, hemophilia can be acquired. "Acquired” means you aren't born with the disorder, but you develop it during your lifetime. This can happen if your body forms antibodies (proteins) that attack the clotting factors in your bloodstream. The antibodies can prevent the clotting factors from working.
Hemophilia can be mild, moderate, or severe, depending on how much clotting factor is in your 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 50% to 100%. People who have severe hemophilia A have a factor VIII activity of less than 1%.
Hemophilia occurs almost exclusively in males (with rare exceptions). About 1 in 7,500 males are born with hemophilia (A or B) each year.