Hemophilia A and hemophilia B are two different subtypes of the rare bleeding disorder hemophilia. Overall, they have a lot in common. For example, they cause similar symptoms and are diagnosed using similar tests. However, more people have hemophilia A than hemophilia B. And the genetics and missing clotting factors are different for each condition.

Understanding what type of hemophilia you or your loved one has is crucial to getting the best possible care for your bleeding disorder. Here are some key differences that set these two conditions apart.

What Is the Main Difference Between Hemophilia A and B?

The biggest difference between hemophilia A and B is the specific clotting factor (coagulation factor) that’s missing. Clotting factors are proteins that help the blood clot in order to prevent excessive bleeding. Insufficient clotting factors put people with hemophilia at risk of uncontrolled bleeding.

People with hemophilia A don’t have enough of the blood-clotting factor VIII (8). This is because they have mutations (changes) in the factor VIII gene. Hemophilia A is also called classical hemophilia. It’s usually inherited or passed down through families. However, about 30 percent of people with hemophilia A don’t have a family history of it. This means the gene mutation happened either soon after they were conceived or at some point during their body’s development.

In hemophilia B, clotting factor IX (9) is the missing factor. It’s the result of mutations in the factor IX gene. Hemophilia B is also known as Christmas disease — it was named after Stephen Christmas, who was the first person diagnosed with it in 1952. Like hemophilia A, hemophilia B also runs in families in most cases. About 30 percent of the time, it develops in people who have no family history.

Hemophilia A Is More Common and More Severe

Hemophilia A is the most common type by far. Between 80 percent and 85 percent of people with hemophilia have hemophilia A. As many as 1 in every 5,000 newborn males has hemophilia A. According to the National Organization for Rare Disorders (NORD), hemophilia B is the second most common type of hemophilia. It affects 1 in every 25,000 newborn males.

According to Cleveland Clinic, women may carry the genes for either type of hemophilia. They can pass it down to their children, but aren’t likely to have severe symptoms themselves.

In 60 percent of cases, hemophilia A symptoms are severe. However, the rate of severe disease in hemophilia B is much lower — about 30 percent.

Severity for both conditions is measured by comparing a person’s clotting-factor level with the typical level. For example, in someone with hemophilia A, clotting factor VIII levels may be:

• 5 percent to 40 percent of the typical amount, which is considered mild
• 1 percent to 5 percent of the typical amount, which is considered moderate
• Lower than 1 percent of the typical amount, which is considered severe

The same breakdown applies to hemophilia B, except that the measurement is for levels of clotting factor IX. People with severe hemophilia are more likely to bleed spontaneously (or without a known injury or cause). They can develop internal bleeding in their joints or brain, leading to life-threatening complications.

Some Treatment Options Differ

Many of the general management tips and treatment options apply to both types of hemophilia. For instance, people with hemophilia should generally avoid taking medications that increase bleeding risk or doing activities that increase their risk of getting injured.

Factor replacement therapy is a standard hemophilia treatment. It involves giving the affected person infusions of the missing clotting factor. People with severe hemophilia receive factor replacement on a regular basis. In milder cases, this treatment is reserved for injuries or before surgery.

Nonfactor treatments are also available for hemophilia A and B, including marstacimab (Hympavzi) and fitusiran (Qfitlia).

Available Gene Therapies

Scientists are developing new gene therapies to boost the body’s natural production of certain clotting factors. Gene therapy aims to eliminate the need for ongoing hemophilia treatment. As of October 2025, valoctocogene roxaparvovec (Roctavian) is the only approved gene therapy for hemophilia A. For hemophilia B, there is a gene therapy available called etranacogene dezaparvovec-drlb (Hemgenix).

Additional Options for Hemophilia A

Mild cases of hemophilia A can also be treated with desmopressin, which is a human-made version of the vasopressin hormone. Desmopressin works by increasing factor VIII levels in the blood. Antifibrinolytics can also be used for mild hemophilia A. These are drugs that slow the breakdown of clotting factors in the blood.

Inhibitors Are Less Likely To Develop in Hemophilia B

One potential complication of hemophilia treatment is the development of inhibitors. Inhibitors are immune proteins the body produces that block factor replacement therapy from working. This means your body is attacking the replacement clotting factor.

People with severe hemophilia A have a 25 percent to 30 percent chance of developing inhibitors during their lifetime. Inhibitor development happens less often in severe hemophilia B, occurring in 3 percent to 5 percent of cases.

Immune tolerance induction may help reduce the effect of inhibitors. It’s the practice of providing replacement factor daily, sometimes for months or years, to eliminate inhibitors.

What About Hemophilia C?

Hemophilia C is also called Rosenthal disease. It’s a very rare form of hemophilia that happens when someone is born missing the factor XI (11) gene. As a result, they don’t produce clotting factor XI. (The Roman numerals can make clotting factors easy to mix up — note the difference between clotting factor XI and clotting factor IX.)

Factor levels can become very low in hemophilia C, but the symptoms can still be mild. It doesn’t cause bleeding problems in the joints and muscles, as seen in hemophilia A and B. It usually presents with excessive bleeding after surgery or trauma.

While the genes for hemophilia A and B are inherited from one biological parent, people with hemophilia C inherit the mutated gene from both parents. According to Cleveland Clinic, it affects men and women equally, and it is more common in those with Ashkenazi Jewish heritage.

If any form of hemophilia runs in your family, a genetic counselor can provide genetic testing and help you understand the risks of passing it on. New treatment options are under development. Your healthcare provider can help keep you in the loop for clinical trials and new therapies as they become available.

Join the Conversation

On MyHemophiliaTeam, people share their experiences with hemophilia, get advice, and find support from others who understand.

What type of hemophilia do you or your loved one have? Let others know in the comments below.