Heterogeneity of human procoagulant factor VIII (VIIIC) antibodies in their reaction with factor VIII related antigen (VIIIRAG).

Human inhibitors to procoagulant factor VIII (VIIIC) were tested for possible reactivity with factor VIII related antigen (VIIIRAg) by a modified VIIIRAg immunoradiometric assay (IRMA). Inhibitor IgG's were screened for anti-VIIRAg by competition with I125 labelled VIIIRAg antibodies for common antigenic determinants using either an VIIIRAg concentrate from which factor VIII coagulant antigen (VIIICAg) had been dissociated, or normal plasma (containing VIIIRAg and VIIICAg) as an antigen source. There was no evidence of VIIIRAg antibodies in the five haemophilic VIIIC inhibitor IgG's tested but low titre VIIIRAg antibodies were detected in one spontaneous VIIIC inhibitor IgG.