Neutrophil adhesion and clustering at different shear stresses. Neutrophils were perfused over a plasma fibrin surface at shear stresses ranging from 20 to 320 mPa. An ECM-coated coverslip was incubated with (leukocyte-free) PPP from heparinized blood at a shear stress of 20 mPa (10 minutes at 37°C). Subsequently, the fibrin surface was used for a perfusion with isolated resting PMNs (2 × 10⁶ cells/mL HEPES buffer). After 5 minutes, the total number of adhering neutrophils per square millimeter (•) and the cluster index (○) was determined. The mean values ± SE of four to six experiments are given.