Occlusion of a mesenteric microvessel by thrombi and fibrinogen deposition on the endothelium. Video-photomicrographs of mesenteric microvessels of rats receiving an intraperitoneally injection of LPS and 3 hours later an infusion of either aPL-positive IgG or aPL-negative IgG. The micrographs were taken 30 minutes after the IgG infusion. Thrombi occluding 2 vessels can be seen in the aPL-positive IgG-treated rats (A) and absence of thrombi in the microvessels of aPL-negative IgG-treated rats (B). To evaluate deposition of fibrinogen, the rats were treated as indicated in “Patients, materials, and methods,” except that a solution of FITC-labeled fibrinogen (3.25 mg/250 μL sterile saline) was infused at the same time as the IgG. The micrographs were taken 10 minutes after the infusion of FITC-fibrinogen. Note the deposition of FITC-fibrinogen on the endothelium of aPL-positive IgG-treated rats (C) as opposed to the absence of staining of the microvessel in rats receiving aPL-negative IgG (D). Original magnification, × 100. Details of figure acquisition are given in Dobrina et al.²³