Hemophilia and Inhibitors


Some people with hemophilia develop antibodies against factor after they have received factor concentrate treatment. These antibodies are referred to as inhibitors.

What are Inhibitors?

An inhibitor is an antibody produced by the body's immune system that:

  • Identifies factor concentrate as a foreign substance
  • Attacks factor
  • Neutralizes factor activity
  • Makes factor ineffective or less effective in stopping bleeding.1

Approximately 30% of individuals with severe hemophilia A—those with factor VIII levels less than 1% of normal—develop inhibitors.2 Only about 1% to 4% of people with severe hemophilia B develop an inhibitor, which is a much lower incidence than in severe hemophilia A.2,3

An inhibitor can develop only after a person with congenital hemophilia has been treated with replacement clotting factor. Most inhibitors occur early in life, usually within the first 50 treatment days with replacement clotting factor.3

What is the Impact of an Inhibitor?

People with inhibitors do not bleed more frequently than other people with hemophilia, but their bleeds are more difficult to manage because they do not always respond to standard treatment.1

Who is at Risk?

Anyone with hemophilia can develop an inhibitor. However, there are some factors that may increase your risk. These include:

  • Major genetic changes, called mutations, in the factor VIII or IX gene4
  • A family history of inhibitors5
  • African heritage5
  • Hemophilia severity5

Learning more about living with inhibitors may help you better understand your condition and take care of yourself. Visit KnowInhibitors.com for facts and tips about living with inhibitors.



  1. Kasper CK. Diagnosis and management of inhibitors to factors VIII and IX. An introductory discussion for physicians. Treatment of Hemophilia. No. 34. Montréal, Canada: World Federation of Hemophilia; 2004.
  2. Who is at Risk for Developing an Inhibitor? National Hemophilia Foundation website. https://www.hemophilia.org/Bleeding-Disorders/Inhibitors-Other-Complications/Inhibitors-for-Consumers/Who-is-at-Risk-for-Developing-an-inhibitor. Accessed January 26, 2017.
  3. Hemophilia B (Factor IX). National Hemophilia Foundation website. https://www.hemophilia.org/Bleeding-Disorders/Types-of-Bleeding-Disorders/Hemophilia-B. Accessed January 26, 2017.
  4. Schwaab R, Brackmann HH, Meyer C, Seehafer J, Kirchgesser M, Haack A. Olek K, Tuddenham EG, Oldenburg J. Haemophilia A: mutation type determines risk of inhibitor formation. Thrombosis and Haemostasis. 1995;74(6)1402-1406.
  5. Who is at risk of developing inhibitors? World Federation of Hemophilia. http://www.wfh.org/en/page.aspx?pid=653. Updated May 2012. Accessed January 26, 2017.

Hemophilia and Inhibitors

Inhibitors are antibodies against factor that develop after receiving factor concentrate treatment. Read about risk factors for inhibitors.