Rapid fibrin formation on ionomycin-stimulated platelets. Adhered platelets were prestimulated with ionomycin (10μM) and CaCl₂ (1mM), then perfused with control plasma (Ctrl) or plasma from a patient with VWD in the presence or absence of 6B4 mAb (10 μg/mL). (A) Representative phase contrast and OG–annexin A5 fluorescence images (Nikon Diaphot 200 microscope, see “Methods”) after ionomycin stimulation (bars, 10 μm). (B) Fractions of type 3 and 4 (procoagulant) platelets before plasma perfusion. (C) Platelet-dependent fibrin formation after 240 seconds. (D) Lag time to fibrin formation after plasma perfusion. Means ± SE; n = 3-4.