Leukocyte adhesion to platelet thrombi and platelet monolayers. Collagen-coated surfaces were exposed for 5.5 minutes to flowing human blood (shear rate, 65/s) that was anticoagulated by infusion of thrombin inhibitor. To obtain collagen-adherent platelet monolayers, platelet aggregation on top of the adherent platelets was prevented by additional infusion of the GPIIb-IIIa antagonist. (A and C) “En face” view of coverslips with platelet thrombi (A) and platelet monolayers (C). P-selectin–expressing platelets (black) were visualized by an immunogold-silver staining using a monoclonal anti–P-selectin antibody, and the adherent leukocytes (blue) were stained with Diff-Quick solution. (B and D) Cross sections of coverslips with platelet thrombi (B) and platelet monolayers (D). Leukocytes can be seen to adhere to the deposited platelet thrombi (in B) and to single collagen-adherent platelets (in D). L, leukocytes; P, single adherent platelet; T, platelet thrombus.