Hemophilia B with inhibitors occurs when the immune system develops antibodies (inhibitors) against the replacement clotting factor IX used to treat the condition.

In typical hemophilia B, individuals have a deficiency of factor IX, a protein essential for blood clotting. To manage and control bleeding episodes, they receive infusions of factor IX replacement therapy. However, for some individuals, the immune system perceives the infused factor IX as a foreign substance and produces inhibitors that neutralize or block its effectiveness. This makes it significantly more challenging to control bleeding, as standard treatments become less effective or ineffective.

Managing hemophilia B with inhibitors often requires alternative therapies, such as bypassing agents (like recombinant factor VIIa or activated prothrombin complex concentrates), immune tolerance therapy, and, more recently, non-factor replacement therapies to bypass or circumvent the need for factor IX directly. This form of hemophilia requires careful management by specialized care teams to reduce bleeding risks and improve the individual's quality of life.