Hemophilia A and B are inherited bleeding disorders in which the blood doesn’t clot properly. The difference comes down to which clotting protein is missing: hemophilia A results from a deficiency of clotting factor VIII, while hemophilia B results from a deficiency of clotting factor IX. Both types cause the same core problem, prolonged or spontaneous bleeding, but they involve different proteins in the clotting chain and can differ in treatment approach and complication rates.
How the Two Types Differ
Your blood contains a cascade of proteins that work together to form clots and stop bleeding. Factor VIII and factor IX are both essential links in that chain. When either one is missing or present at very low levels, the chain breaks down and bleeding continues longer than it should.
Hemophilia A, sometimes called classic hemophilia, is the more common form. It accounts for roughly 80% of all hemophilia cases. Hemophilia B, also known as Christmas disease (named after the first patient described with it, not the holiday), makes up most of the remaining cases. Despite involving different clotting factors, the two types produce nearly identical symptoms, and most people can’t tell them apart without blood testing.
Severity Levels
Both hemophilia A and B are classified into three severity levels based on how much functional clotting factor is circulating in the blood. These categories, established by the International Society on Thrombosis and Hemostasis, directly predict a person’s bleeding pattern:
- Severe: Less than 1% of normal factor activity. People with severe hemophilia often bleed spontaneously, particularly into joints and muscles, without any obvious injury.
- Moderate: 1% to 5% of normal factor activity. Bleeding episodes are less frequent but can still occur with minor trauma or, occasionally, on their own.
- Mild: Greater than 5% but less than 40% of normal factor activity. Bleeding typically only becomes a problem after surgery, dental work, or significant injury. Some people with mild hemophilia don’t know they have it until a medical procedure triggers unusual bleeding.
A person’s severity level stays consistent throughout life because it’s determined by genetics, not by age or lifestyle.
Why Males Are Affected More Often
The genes for both factor VIII and factor IX sit on the X chromosome. Males have one X and one Y chromosome, and the Y chromosome carries no clotting factor genes. That means a male with a faulty gene on his single X chromosome has no backup copy, so he will have hemophilia.
Females have two X chromosomes, so a working gene on one X can often compensate for a faulty gene on the other. A woman who carries one hemophilia gene is called a carrier. Carriers can pass hemophilia to their children, and some carriers do experience bleeding symptoms themselves, though these are usually milder than in males. In rare cases, a carrier’s symptoms can be just as severe, particularly if her second X chromosome is also affected or isn’t functioning properly.
A woman can have full hemophilia if she inherits a hemophilia gene from both parents, or if she has one hemophilia gene and her other X chromosome is missing or inactive. These situations are uncommon but well documented.
Symptoms and Joint Damage
The hallmark symptom of hemophilia is bleeding that lasts longer than expected. Cuts and scrapes may bleed more, but the more dangerous bleeding happens inside the body where you can’t see it.
Joint bleeding is the most characteristic complication, especially in severe hemophilia. Blood leaks into the synovial membrane, the thin protective lining inside joints like the knees, ankles, and elbows. This causes swelling, warmth, stiffness, and pain. A single bleed into a joint can be managed, but repeated bleeds into the same joint over months or years damage the cartilage and bone, eventually leading to chronic pain, limited range of motion, and arthritis-like degeneration.
Other symptoms include deep muscle bleeding (which can cause pressure on nerves), excessive bruising, prolonged bleeding after dental work or surgery, and in serious cases, bleeding into the brain. Nosebleeds and blood in the urine or stool can also occur.
How Hemophilia Is Diagnosed
Diagnosis starts with blood tests that measure how long it takes blood to clot. If screening tests suggest a clotting problem, a follow-up test called a factor assay measures the exact level of factor VIII and factor IX in the blood. This test identifies which type of hemophilia is present and determines the severity level. Families with a known history of hemophilia can also pursue genetic testing to identify carriers or diagnose newborns early.
Treatment: Prophylaxis vs. On-Demand
The primary treatment for both types is replacing the missing clotting factor. How that replacement is delivered depends on severity and individual circumstances, but it falls into two broad strategies.
Prophylactic treatment means receiving regular infusions of clotting factor on a set schedule, typically several times a week, to keep factor levels high enough to prevent bleeds before they happen. When started early in life, prophylaxis has been associated with over 90% reduction in joint bleeding rates and significantly less long-term joint damage. This is the standard of care for people with severe hemophilia.
On-demand (episodic) treatment means infusing clotting factor only when a bleed occurs. This approach reduces pain and the immediate impact of individual bleeds, but it doesn’t change the overall bleeding pattern over time. Joints still accumulate damage with each episode. On-demand therapy is essential when prophylaxis isn’t available, but it doesn’t prevent the progressive musculoskeletal problems that define severe hemophilia.
Newer Treatment Options
For hemophilia A specifically, a newer class of treatment works differently from traditional factor replacement. Instead of supplying the missing factor VIII directly, one widely used option is a bispecific antibody that mimics what factor VIII does: it bridges two other clotting proteins together to keep the clotting cascade moving. This treatment is given as an injection under the skin rather than into a vein, and it’s effective in people both with and without inhibitors (a complication discussed below). It’s used only for hemophilia A, not B, because it specifically mimics factor VIII’s role.
Gene therapy has also reached approval for both types of hemophilia. The FDA has approved gene therapies for hemophilia A and hemophilia B. These one-time treatments deliver a working copy of the relevant gene so the body can produce its own clotting factor. Gene therapy doesn’t work for everyone, and its long-term durability is still being tracked, but for some adults it has reduced or eliminated the need for regular infusions.
The Inhibitor Problem
One of the most serious complications of hemophilia treatment is the development of inhibitors. These are antibodies the immune system produces against the infused clotting factor, essentially treating the medication as a foreign invader and neutralizing it. When inhibitors develop, standard factor replacement therapy stops working effectively.
This complication is significantly more common in hemophilia A: roughly 1 in 5 people with hemophilia A develop inhibitors, compared to about 3 in 100 people with hemophilia B. Inhibitors typically appear within the first 50 exposure days to factor replacement, so they’re most often detected in young children. Treatment options exist for people with inhibitors, including bypassing agents that work around the missing factor, immune tolerance therapy designed to train the immune system to accept the clotting factor, and the newer bispecific antibody approach for hemophilia A.
The difference in inhibitor rates between the two types is one of the most clinically meaningful distinctions between hemophilia A and B, and it’s a key reason treatment plans are tailored to each type individually.