Precipitating anti-VIII:C antibodies in two patients with haemophilia A.

IgG from the plasmas of two haemophilia A patients with anti-VIII:CAg antibodies (1000 and 200 u/ml) was isolated and labelled with 125I. The specific labelled anti-VIII:CAg IgG was further purified by binding to and elution from immobilized factor VIII/von Willebrand factor (F.VIII/vWF). When studied by immunodiffusion and autoradiography, both antibodies gave a precipitin line with normal plasma, serum, cryoprecipitate, purified F.VIII/vWF and the plasmas of two patients with haemophilia A+. No precipitin line was observed with the plasmas of 11 patients with haemophilia A- or four patients with severe von Willebrand's disease. Levels of VIII:CAg obtained by radioelectroimmunoassay were in agreement with those obtained by immunoradiometric assay. This study demonstrates that, contrary to previous evidence, human anti-VIII:CAg antibodies are precipitating as well as neutralizing when studied by highly sensitive techniques.