Pathogenesis of TTP. Shown are endothelial cell cultures stimulated to release VWF multimers perfused with fluorescence-labeled platelets suspended in a plasma with a severe ADAMTS13 deficiency of < 5% of the normal (top panel) or in a plasma with normal ADAMTS13 activity (bottom panel). Experiments were performed by Dr R. Fröhlich-Zahnd in the laboratory of Prof J.-F. Dong (at the time with the Department of Medicine, Baylor College of Medicine, Houston, TX, and now at the Puget Sound Blood Center, Seattle, WA).^47,48