Studies on the stability of factor VIII coagulant antigen (VIII:CAg) in the presence of VIII:C antibodies.

The stability of factor VIII coagulant antigens (VIII:CAg) at 56 degrees C was investigated using an immunoradiometric assay for VIII:CAg. In normal or CRM+ haemophilic plasmas VIII:CAg was rapidly inactivated at 56 degrees C. VIII:CAg in spontaneous VIII:C inhibitor plasmas and in post-treatment samples from haemophiliacs with VIII:C inhibitor was resistant to inactivation at 56 degrees C, indicating the presence of heat stable VIII:CAg-anti VIII:CAg complexes. In vitro VIII:CAg-anti VIII:CAg complexes were formed by incubation of diluted VIII:C antibodies and normal plasma and the stability of these complexes at 56 degrees C was studied. Haemophilic VIII:CAg antibodies formed heat stable immune complexes over a narrow range of inhibitor dilutions whilst some spontaneous VIII:CAg antibodies formed these stable complexes over a much wider range of dilutions emphasizing the difference in the properties of these antibodies.