Effect of IgG on the interaction between factor IXa and factor VIII light chain. Various concentrations of purified IgG from the inhibitor patient (•) and a healthy control (○) were incubated with factor VIII light chain immobilized by MoAb CLB-CAg 12 on a microtiter plate. After 2 hours incubation at 37°C the IgG was removed and 40 nmol/L factor IXa was added. Residual factor IXa bound was determined after 4 hours of incubation at 37°C and calculated relative to a control experiment in which no IgG was added. Each datapoint represents the mean (±SD) of at least three experiments.