Factor Xa-antithrombin III (FXa-AT-III) interactions. (A) Effect of IgG on FXa activity in the presence of AT-III (20 nM). Bars represent the mean ± standard error of the mean of all individual IgG from each relevant group; ****P <0.0001. (B) Inhibition of AT-III inactivation of FXa by antiphospholipid syndrome (APS)-IgG and AT-III binding of APS-IgG. There is no correlation of AT-III binding of APS-IgG with inhibition of AT-III inactivation of FXa by APS-IgG. Dots represent individual APS-IgG samples. Experiments were performed in duplicate for each sample. Results are representative of at least three independent experiments.