Annexin A5 binding studies. (A) Effects of spiking plasmas with aPL IgG on annexin A5 binding to phospholipid. aPL or control IgG was added to a normal plasma to a final concentration of 7.5 mg/mL and incubated with aPTT reagent-phospholipid. The phospholipid preincubated with aPL IgG-spiked plasma bound significantly less labeled annexin A5 (mean ± SD, 12.1 ± 2.6 ng/aliquot of PL) compared with the phospholipid preincubated with control IgG-spiked plasma (16.7 ± 0.7 ng/aliquot of PL, P < .05) (n = 3 pairs of different IgG). (B) Effects of depleting aPL IgG from plasmas on annexin A5 binding. aPL plasmas that were pretreated with protein G-Sepharose significantly increased the ability of annexin A5 binding to phospholipid (mean ± SD, 8.4 ± 2.5 ng/aliquot of PL) compared with aPL plasmas preincubated with Sepharose 2B (5.8 ± 2.4 ng/aliquot of PL, P < .05). In control plasmas, no significant difference on the binding of annexin A5 was observed between plasmas preincubated with protein G-Sepharose (9.0 ± 1.3 ng) and plasmas pretreated with Sepharose 2B (10.1 ± 1.9 ng) (n = 10 plasmas for each group). NS indicates not significant. (C) Effects of varying concentrations of aPL plasmas on annexin A5 binding. aPL plasmas were serially diluted in a normal plasma and incubated with aPTT reagent-phospholipid. The quantity of annexin A5 bound to phospholipid decreased progressively as the concentration of aPL plasma, diluted in a normal control plasma, was increased (each point shows the mean of 2 aPL patient plasmas).