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

S ummary . The stability of factor VIII coagulant antigens (VIII:CAg) at 56°C was investigated using an immunoradiometric assay for VIII: CAg. In normal or CRM⁺ haemophilic plasmas VIII: CAg was rapidly inactivated at 56°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°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°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.