Failure of immunosuppression in a severe haemophilia B patient with specific antibody.

Prevention of a secondary response to factor IX by cyclophosphamide was attempted in an 11 year old patient with severe Christmas disease. An antibody to factor IX had been present for 4 years before immunosuppressive therapy was tried. Despite profound lymphopenia, synthesis of factor IX antibody was not depressed. The difficulties of modifying the anamnestic response to factor IX by chemical immunosuppression may be as real as has been reported for factor VIII in classical haemophilia.