Free Hb enhances the interaction between platelet GPIbα and purified plasma VWF at high shear stress. (A,C) Citrated whole blood with buffer or (B,D) purified human HbA (500 mg/dL) was perfused over a surface coated with purified plasma VWF or wild-type A1A2A3 protein at high shear stress (60 dyne/cm²). A magnification of ×10 was used for VWF, whereas ×20 was used for A1A2A3 protein. Platelets from blood containing free Hb were quickly arrested and capable of forming stable microthrombi.