Hemophilia A and hemophilia B are both inherited bleeding disorders caused by a shortage of clotting proteins in the blood, but they involve different proteins. Hemophilia A results from a deficiency in clotting factor VIII, while hemophilia B results from a deficiency in clotting factor IX. This single distinction drives meaningful differences in how common each type is, how treatment works, and what complications to expect.
The Core Difference: Two Different Clotting Factors
Your blood relies on a cascade of proteins to form clots and stop bleeding. Factor VIII and factor IX are both essential links in that chain, and losing either one disrupts the process in a similar way. People with hemophilia A are missing or have low levels of factor VIII. People with hemophilia B are missing or have low levels of factor IX. The end result, excessive or prolonged bleeding, looks the same in both types. Without specific blood tests measuring each factor’s activity level, it’s impossible to tell the two apart based on symptoms alone.
How Common Each Type Is
Hemophilia A is far more common. In the United States, it occurs in roughly 1 out of every 5,617 male births. Hemophilia B is about three to four times rarer, affecting approximately 1 in 19,283 male births. Both types overwhelmingly affect males because the genes for factor VIII and factor IX both sit on the X chromosome. Women who carry one affected X chromosome are usually protected by their second, normal copy, though some carriers do experience mild bleeding symptoms.
Severity Works the Same Way for Both
Doctors classify both hemophilia A and B using the same scale, based on how much working clotting factor your blood actually contains compared to normal levels:
- Severe: less than 1% of normal factor activity. Spontaneous bleeding into joints and muscles can happen without any injury.
- Moderate: 1% to 5% of normal. Bleeding episodes are less frequent but can occur with minor trauma or sometimes spontaneously.
- Mild: above 5% but below 40%. Bleeding is typically only a problem after surgery, dental work, or significant injury. Some people with mild hemophilia don’t discover their condition until adulthood.
A person with severe hemophilia A and a person with severe hemophilia B will have very similar day-to-day experiences. The severity level matters more for quality of life than whether you have type A or type B.
Symptoms Are Nearly Identical
Both types produce the same bleeding patterns. The hallmark is bleeding into joints, especially the knees, ankles, and elbows, which causes swelling, pain, and over time can lead to permanent joint damage. Deep muscle bleeding, prolonged bleeding after cuts or dental procedures, and easy bruising are common to both. In severe cases, bleeding can occur inside the skull or other internal organs, which is a medical emergency regardless of type.
Because the symptoms overlap completely, the only reliable way to distinguish hemophilia A from B is through lab work that measures factor VIII and factor IX levels separately.
Treatment Differences
The primary treatment for both types is replacing the missing clotting factor through infusions. People with hemophilia A receive factor VIII concentrate, and people with hemophilia B receive factor IX concentrate. These can be given on demand (to stop active bleeding) or on a regular preventive schedule called prophylaxis.
One practical difference is how often infusions are needed. Factor VIII, once infused, has a half-life of about 12 hours, meaning it loses half its effectiveness in that time. Factor IX lasts longer in the bloodstream, with a half-life of 18 to 24 hours. This means people with hemophilia B may be able to space their preventive infusions slightly further apart than those with hemophilia A, though newer extended half-life products have improved the picture for both types.
A significant treatment advantage currently exists for hemophilia A. A preventive therapy called emicizumab, given as an injection under the skin rather than an intravenous infusion, is approved specifically for hemophilia A. It works by mimicking the function of factor VIII in the clotting cascade, and it can be used whether or not a patient has developed complications from standard treatment. No equivalent non-factor therapy is currently approved for hemophilia B, so people with type B still rely primarily on factor IX infusions.
Inhibitor Risk Is Higher in Type A
One of the most serious complications of hemophilia treatment is the development of inhibitors. These are antibodies your immune system produces against the replacement clotting factor, essentially treating the medicine as a foreign invader and neutralizing it. When inhibitors develop, standard factor replacement stops working effectively.
This problem is much more common in hemophilia A. About 1 in 5 people with hemophilia A will develop inhibitors, according to the CDC. For hemophilia B, the rate is roughly 3 in 100. This is a meaningful clinical difference: hemophilia A patients face a substantially higher chance of needing alternative treatment strategies to manage their bleeding.
Gene Therapy
Gene therapy has moved from experimental to approved for both types, though hemophilia B reached the milestone first. The FDA approved Hemgenix, a gene therapy for hemophilia B, making it the first gene therapy for any type of hemophilia to receive regulatory approval in the United States. Gene therapy for hemophilia A has also been approved. Both work by delivering a functional copy of the missing gene so the body can produce its own clotting factor, potentially reducing or eliminating the need for regular infusions.
Hemophilia B has been somewhat easier to target with gene therapy because the factor IX gene is smaller and simpler to package into the viral delivery systems used in these treatments. The factor VIII gene is one of the largest in the human genome, which presented greater technical challenges for researchers.
How They Compare at a Glance
- Missing factor: Factor VIII in type A, factor IX in type B
- Prevalence: Type A is about 3 to 4 times more common
- Symptoms: Identical for the same severity level
- Infusion frequency: Slightly more frequent for type A due to shorter factor half-life
- Inhibitor risk: Roughly 20% for type A versus 3% for type B
- Non-factor preventive therapy: Available for type A (emicizumab), not yet for type B
Despite these differences, the day-to-day experience of living with either type is shaped far more by severity level than by which factor is missing. Someone with mild hemophilia B and someone with mild hemophilia A will have far more in common with each other than either will with someone who has the severe form of their own type.