Factor IX antibody and immune tolerance.

Prevalence of FIX inhibitor is ten times lower than factor VIII inhibitor. FIX inhibitor patients pose major challenges for treatment because of the simultaneous occurrence of severe allergy to FIX at the time of inhibitor development. Immune tolerance induction in haemophilia B inhibitor patients with allergy to FIX is complicated due to the development of nephrotic syndrome. Moreover, response to ITI usually is poor.