In its most severe form, haemophilia A is a life-threatening haemorrhagic bleeding disorder that is caused by mutations in the factor VIII (FVIII) gene. About 25% of patients who receive replacement therapy with intravenous FVIII products develop neutralising antibodies (FVIII inhibitors) that inhibit the function of substituted FVIII. Long-term application of high or low doses of FVIII has evolved as an effective strategy for eradicating antibodies and inducing long-lasting immune tolerance. Despite clinical experience with the therapy, little is known about the immunological mechanisms that cause the down modulation of FVIII-specific immune responses or the induction of long-lasting immune tolerance against FVIII. This review summarises current knowledge of the immunological mechanisms that might be involved in the induction of immune tolerance against FVIII in patients with haemophilia A who have FVIII inhibitors. In addition to data from patients with haemophilia A, data from patients who have had organ transplants or have immune-related disorders, such as autoimmune diseases, are considered as well as data from animal models.
