People with hemophilia can have uncontrolled or even spontaneous bleeding. Most people with hemophilia lack clotting factor VIII (8), known as classic hemophilia, or hemophilia A. A less familiar type is hemophilia B — also called Christmas disease. In this condition, factor IX (9) is missing or defective.

This rare disease shares similarities with other forms of hemophilia. But factor IX deficiency also has some key differences, like where it comes from and how it responds to treatment.

Here are four notable — and possibly surprising — facts about hemophilia B.

1. Christmas Disease Refers to a Person

Christmas disease is named after the first person diagnosed with hemophilia B. He was a 5-year-old boy named Stephen Christmas. Born in England in 1947, the child had bruised easily as a baby and had been hospitalized nine times for hematomas. Hematomas occur when blood (typically clotted) collects outside a vessel.

Doctors initially diagnosed him with a variety of conditions, including classic hemophilia. But researchers looking into his case found that he lacked blood clotting factor IX, not VIII. They reported their findings in the 1952 British Medical Journal study “Christmas Disease: A Condition Previously Mistaken for Haemophilia.”

Why such a relatively recent discovery, when doctors had known about hemophilia since the early 1800s? The condition might have gone unidentified since hemophilia B is rarer than other types of hemophilia. According to the Centers for Disease Control and Prevention (CDC), hemophilia B occurs in only about 1 in 19,200 male births in the United States. This is compared with about 1 in 5,617 births for the most common type. Like most hemophilias, hemophilia B affects males much more than females.

2. Hemophilia B Tends To Be Milder Than Other Types

As with all types of hemophilia, symptoms can range from mild to severe in hemophilia B. But newer studies have shown that people with hemophilia B are more likely to have milder cases than those with other types.

People with milder forms may have few or no symptoms. In addition, they may need pretreatment only before surgery or another invasive procedure. Those with more severe forms of hemophilia B are at risk of hemorrhage (life-threatening and sudden bleeds). Hemorrhages may affect the joints, gastrointestinal tract, or central nervous system, and they require frequent treatment to prevent excessive bleeding. But researchers have reported that people who have hemophilia B are less likely to bleed than those with classic hemophilia. This is true even among people with moderate to severe hemophilia B.

3. Hemophilia B Responds Better to Factor Treatment

All hemophilias involve the body’s shortage of a specific clotting factor. For that reason, treatment involves infusions to administer the insufficient factor. In the case of hemophilia B, that’s factor IX.

One reason the disease is generally milder may be that those with hemophilia B have less of a tendency to develop inhibitors. Inhibitors are antibodies that target and destroy infused factors. Only 3 out of 100 people with hemophilia B develop an inhibitor compared with an estimated 20 out of 100 people who develop an inhibitor with classic hemophilia.

An inhibitor can destroy infused clotting factors before they have a chance to work. Since most people with hemophilia B don’t develop inhibitors, their treatment is more likely to succeed.

4. Many People With Hemophilia B Didn’t Inherit It

Hemophilia is a genetic condition — it’s caused by mutations (changes) in genes. These mutations can be sporadic (acquired at random) or passed on from parents. About one-third of males born with hemophilia B have no family history of hemophilia, according to the journal The Application of Clinical Genetics.

Hemophilia B, like other hemophilias, is an X-linked recessive condition. This means it’s carried only on an X chromosome, not a Y chromosome. Males are typically born with an X and a Y chromosome, whereas females are typically born with two X chromosomes. Having a second X chromosome essentially balances out the one with the mutation.

But people with both an X and Y chromosome who carry hemophilia B genes can also experience symptoms. These can occur if the person inherited hemophilia from both parents or if the nonmutated X chromosome is inactive. Individuals with abnormal or prolonged bleeding episodes should be evaluated for hemophilia.

In adults, these mutations can be corrected through gene therapies approved by the U.S. Food and Drug Administration (FDA). These gene therapies, such as etranacogene dezaparvovec-drlb (Hemgenix), help people produce more factor IX.

If gene therapy isn’t right for you, your healthcare provider may recommend specific medications. For instance, they may suggest medications like concizumab-mtci (Alhemo). This drug promotes blood clotting through a different pathway that doesn’t involve factor IX. Concizumab-mtci is approved by the FDA for people 12 and older who have hemophilia A or B with or without inhibitors.

Work With Your Medical Team

Hemophilia B can be a complex condition to understand. Be sure to bring up any questions you have during visits with your hematologist (blood disorder specialist).

If you’ve been diagnosed with hemophilia B, it’s important to keep up regular appointments with your hematologist. They can work with you to create an effective treatment plan. They’ll take factors into account, such as the severity of your disease, your genetics, and if you have an inhibitor. These medical experts in bleeding disorders will discuss treatment options with you so you can share the decision-making and develop a plan that’s right for you.

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