Effects of infusion of human aPL-positive and aPL-negative IgG in rats with or without pretreatment with LPS. Human IgG (10 mg/1 mL sterile saline) purified from 6 aPL antibody-positive sera was infused into the carotid artery of Wistar rats 3 hours after the intraperitoneally injection of either LPS (□) or sterile saline (▧). Another group of rats was treated with 5 aPL-negative IgG (10 mg/1 mL sterile saline) with (▪) or without (▦) pretreatment with LPS. The procoagulant effect of the various treatments was evaluated by counting the number of microvessels with partial or total occlusions (A) and the number of occluded vessels, as shown by the complete and persistent stop of the blood flow (B). These parameters were evaluated on 2 rats for each IgG sample to a total number of 12 and 10 rats for the aPL-positive and aPL-negative IgG, respectively. The results are expressed as mean ± SD. ^**P < .01, ^*P < .05 versus control rats receiving aPL-negative IgG.