Random and clustered distribution of neutrophils adhered to fibrin. Neutrophils were perfused over a plasma fibrin surface as described in Fig 1 and pictures were taken after 0, 1, 3, and 5 minutes of perfusion. (A) Clear cellular clusters, aligned in the direction of the flow, occur after a perfusion at a shear stress of 160 mPa. Under these conditions, the mean cluster index after 5 minutes of perfusion is approximately 1.8. (B) More neutrophils adhered in a more random fashion after a perfusion at a low shear stress of 20 mPa. Under these conditions, the mean cluster index after 5 minutes of perfusion is approximately 0.5. The experiments shown are representative of four additional experiments. The arrow indicates the direction of the flow (equal for all figures).