Inactivation of factor Va activity by APC in the presence of control or patient’s IgG. Factor Va was incubated with APC under various conditions. At different time points, samples were removed from the various incubation mixtures, kept at 37°C, and residual factor Va measured. Each point represents the mean of two experiments. (A) To TBS-BSA containing 5 mmol/L CaCl₂ and 3 mg/mL of either control IgG (▪) or patient’s IgG (◂) factor Va at a final concentration of 12 nmol/L was added. This was followed by immediate addition of APC at a final concentration of 40 nmol/L. (B) Incubation mixtures of TBS-BSA containing 5 mmol/L CaCl₂, 5% vol/vol phospholipid (PL), control (▪) or patient’s IgG at concentrations of 0.2 mg/mL (◂) or 3.7 mg/mL (•), 3 nmol/L factor Va, and 0.2 nmol/L APC. The curves for the two concentrations of control IgG were almost identical, and the curve shown represents the average of the data points. (C) Shows experiments as in (B) except for addition of protein S at a final concentration of 40 nmol/L immediately before the addition of factor Va and APC.