How does an American bison protect itself?

By using their massive body size and sharp horns and hooves. They also use the fight or flight response to protect themselves. Bison can run quite fast for their bulky size. A bull or cow will charge any predator that comes too close, and can even turn on a dime, faster than a human can react to get out of trouble. Since bison are prey animals, they would mostly choose to run away from a predator than to stand an fight, but there can be exceptions to this rule, depending on the circumstances and the predator[s] that is/are threatening them. Bison will also form a bit of a ring against predators if there are young calves that need protection.