Inhibitors are antibodies that the immune system develops because it sees the infused clotting factor as a foreign substance that needs to be destroyed. Antibodies are proteins that eat up the activated factor before it has time to stop the bleeding.

Approximately 30% of people with severe hemophilia A are affected by inhibitors at some point in their lives. While people with severe hemophilia are more likely to develop inhibitors, approximately 5-8% of people with mild or moderate hemophilia A develop inhibitors. Approximately 2-3% of those with hemophilia B develop inhibitors.  

An inhibitor usually occurs between the 5th and 50th infusion of factor concentrate, but any person with a bleeding disorder can develop an inhibitor at any time.

In addition to the type and severity of hemophilia someone has, there are other variables that can increase the risk of inhibitor development:

  • age/number of exposures to factor product
  • family history of an inhibitor
  • race/ethnicity (people of African American and Hispanic descent are at a higher risk of developing an inhibitor)
  • gene mutation (there are ongoing studies that indicate the type of genetic mutation one has may indicate a higher risk of inhibitor development)
  • intensive factor therapy related to surgery or trauma

Treating people with an inhibitor is complex and remains one of the biggest challenges in hemophilia care today. Inhibitors that persist can increase the risks associated with bleeding episodes because factor therapy becomes less effective. After a new treatment schedule is established, the blood’s factor level must be measured to see if the inhibitor goes away, or is tolerized. If feasible, a person with an inhibitor should be cared for under the supervision of a Hemophilia Treatment Center (HTC).

The RID facebook page ( is a great resource for those with or affected by inhibitors.