Start with the basics: hemophilia 101
Interested in learning about hemophilia and how it happens? You’re in the right place.
Zihao lives with hemophilia A.
What is hemophilia?
Hemophilia is a bleeding disorder where the blood doesn’t clot properly. The disease results from a missing or deficient protein, known here as a factor, needed for blood clotting. Without the functioning clotting factor, blood can’t clot as it normally would.
How do people get hemophilia?
Hemophilia is usually genetic—something you’re born with. However, one third of cases are caused by a spontaneous mutation, meaning it happens by chance. In rare cases, a person can develop hemophilia later in life. This is known as acquired hemophilia.
The two main types of hemophilia
Hemophilia is a bleeding disorder caused by low levels of clotting factor in the blood. While there are many types of bleeding disorders, hemophilia usually refers to these two:
Hemophilia A
Caused by low levels of clotting Factor VIII
About 80% of people with hemophilia have this type.
Hemophilia B
Caused by low levels of clotting Factor IX
It affects the remaining 20%
Understanding severity levels
Hemophilia is categorized as mild, moderate, or severe based on how much clotting factor is in the blood. This chart shows how those levels affect bleeding risk and day-to-day life.
Type
Factor level
What it means
Mild
Factor Level
5–40%
of normal factor level activity
What it means
Mild hemophilia means a factor VIII or IX level ranging from 5% up to 40% of normal factor level activity.
May bleed more after serious injury or surgery, but only rarely experiences spontaneous bleeding.
Moderate
Factor Level
1–5%
of normal factor level activity
What it means
Moderate hemophilia refers to a factor VIII or IX level ranging from 1% up to 5% of normal factor level activity.
Bleeding often follows minor injuries.
Severe
Factor Level
Less than 1% of
normal factor level activity
What it means
Severe hemophilia means a person has a factor VIII or IX level below 1% of normal factor level activity.
Bleeds can happen spontaneously and frequently.
Note
Bleeding patterns can vary. Talk to your care team about what’s typical for you.
The bleeding disorders spectrum
Hemophilia A and B are the most well-known—but they’re not the only bleeding disorders. In fact, there are more than 17 identified bleeding disorders. Each one affects blood clotting in a different way.
Click on the bleeding disorder type to learn more about it:
Factor VIII Deficiency
(Hemophilia A)
Caused by low levels of factor VIII (factor 8). It mostly affects males and is usually inherited from the mother, though about one-third of cases result from a spontaneous gene mutation.
Factor IX Deficiency
(Hemophilia B)
Caused by low levels of factor IX (factor 9).
It’s inherited, affects mostly males, and occurs in about 1 in 30,000 live male births.
Factor XI Deficiency
(Hemophilia C)
People with factor XI deficiency (factor 11 deficiency) have low levels of factor XI in their blood. Factor XI deficiency occurs in 1 in every 100,000 people, is inherited from either parent, and occurs in both men and women.
Acquired
Hemophilia (AH)
Unlike inherited types, AH develops later in life in people with no bleeding disorder history. It’s rare and happens when the immune system creates an antibody that blocks clotting—often against factor VIII or XI.
Glanzmann’s
thrombasthenia
A rare disorder where a protein that helps platelets stick together is missing or doesn’t work. GT is inherited from both parents and affects both men and women.
Bernard-Soulier
syndrome
Bernard-Soulier syndrome is an inherited condition in which defective receptor prevents platelets from sticking to injured blood vessel walls. It affects both men and women.
Platelet storage
pool deficiency
Platelet Storage Pool Deficiency (PSPD) is the name given to several rare bleeding disorders caused by a deficiency in platelet granules. With PSPD there may not be enough of a certain type of granule, the granule may be abnormal, the granule may not contain enough of the chemicals, or the platelets cannot release the chemicals from the granules. These abnormalities mean the blood takes longer to form a clot.
Factor I
(fibrinogen) deficiency
Factor I deficiency means a person has problems with a protein called fibrinogen, which is needed to help the body form the initial plug after injury. Factor I deficiency includes afibrinogenemia, hypofibrinogenemia, dysfibrinogenemia, and hypofibrinogenemia, and occurs in 1-2 people per million.
Afibrinogenemia
Afibrinogenemia means a person has no factor I (fibrinogen) in their blood, so their body can’t form a stable clot. It is inherited from both parents and can occur in both men and women.
Hypofibrinogenemia
A person with hypofibrinogenemia has lower levels of factor I (fibrinogen) in their blood, so their body can’t form a stable clot. It affects both men and women and is usually inherited from both parents.
Dysfibrinogenemia
Dysfibrinogenemia means that a person has normal levels of factor I (fibrinogen), but it doesn’t work as it should. It occurs equally in men and women and is usually inherited from one parent, but can also develop as a person gets older.
Factor II
deficiency
With Factor II deficiency (factor 2 deficiency), a person lacks enough factor II in their blood to form a stable clot. It is one of the rarest inherited bleeding disorders. Studies have shown a higher incidence of factor II deficiency among Latinos.
Factor V
deficiency
People with Factor V deficiency (factor 5 deficiency) have low levels of factor V in the blood so their bodies cannot form a stable clot to stop a bleed. It occurs in 1 in every 1 million people, is inherited from both parents, and affects both men and women.
Factor VII
Deficiency
Factor VII deficiency (factor 7 deficiency) is a rare bleeding disorder affecting 1 in 300,000–500,000 people. Caused by low levels of factor VII, it’s inherited from both parents and affects men and women equally.
Factor X
deficiency
Factor X deficiency (factor 10 deficiency) means a person has low levels or factor X is not working properly in their blood. It occurs in 1 in every 500,000 to 1 million people. It is inherited from both parents or developed later in life as an acquired deficiency.
Factor XIII
Deficiency
Factor XIII deficiency (factor 13 deficiency) is an extremely rare disorder—affecting 1 in 5 million people—caused by low levels of factor XIII. It affects both men and women and is usually inherited from both parents, though it can also develop later in life.
von Willebrand Disease
(vWD)
The most common bleeding disorder, affecting 1 in 100 people. Caused by low or malfunctioning von Willebrand protein, it’s inherited from one or both parents and affects men and women equally.
What are inhibitors?
In some people with hemophilia, the body reacts to treatment by developing inhibitors—antibodies against the clotting factor that can destroy it before it has a chance to work.
Inhibitors aren’t present at birth. They tend to appear after someone starts factor replacement therapy, often within the first 50–75 exposure days, usually during childhood.
It’s a defense response.
The immune system sees the infused factor as something unfamiliar and tries to fight it—just like it would with a virus or bacteria. In doing so, it creates inhibitors that stop the treatment from working.
Not everyone develops inhibitors, but certain factors increase the chance:
• Type of hemophilia: More common in hemophilia A
• Severity: More likely in people with severe hemophilia
• Family history: A known history of inhibitors raises your risk
• Ethnicity: More common in people of African descent
• Treatment timing: High-intensity factor treatment early on can be a trigger
What are bypassing agents?
When standard factor treatment isn’t effective due to inhibitors, bypassing agents can help. These treatments work around the missing factor—letting the blood clot without needing Factor VIII or IX.
Bypassing agents come in two types:
Plasma-derived
made from donated human plasma
Recombinant
synthetically engineered in the lab
Navigating ahead
There’s more to hemophilia than just the science.
From daily routines to long-term planning, discover tips and tools to help you navigate your own path.
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