A 25-year-old machinist is injured by a metal sliver in his left hand. Over the next few days, the wounded area becomes reddened, tender, swollen, and feels warm to the touch. The patient goes to the emergency room to have the sliver removed. Twenty-four hours later, endothelial cells at the site of injury in the patient release a chemical mediator that inhibits further platelet aggregation. Name this mediator of inflammation.