F10 and systemic lupus erythematosus: In around 50% of patients with SLE and/or APS, increased levels of anti(a)Thr and antiactivated factor X (aFXa) IgG have been identified and shown to have inhibitory effects on inactivation of procoagulant SP and functional activities of anticoagulant/fibrinolytic SP.12 The potential effect on complement activation has not been previously addressed in vitro.