Highly variable bleeding times and protection from collagen/epinephrine–induced thromboembolism in α_2A^–/– mice. (A) Mice were anesthetized, and a 3-mm segment of the tail tip was cut off with a scalpel. Tail bleeding times of wild-type (n = 26) and α_2A^–/– (n = 31) mice were monitored by gentle absorption of a drop of blood. When no blood was observed on the paper after a 15-second interval, bleeding was determined to have ceased. The experiment was stopped after 20 minutes. Each symbol represents one mouse, and horizontal lines indicate quartiles of bleeding time. (B) Thromboembolic death was observed after the injection of collagen (0.8 mg/kg) and epinephrine (60 μg/kg). All wild-type mice (23 of 23) died, whereas 81.5% (22 of 27) of α_2A^–/– mice and all mice injected with FcRγ-chain^–/– (5 of 5) survived.