Binding of hirudin to arterial sections. Frozen sections were prepared from carotid arteries of HCII^−/− (A,C) or HCII^+/+ (B,D) mice harvested 1 hour after injury. The sections were then incubated with hirudin in vitro, and bound hirudin was detected with sheep anti–hirudin IgG and a secondary antibody linked to peroxidase (C,D). Controls in which the primary antibody was omitted are shown (A,B). A thrombus is visible in the arterial lumen of the HCII^−/− mouse. IEL indicates internal elastic lamina; EEL, external elastic lamina.