What do rattlesnakes do to defend themselves?

By injecting venom through hypodermic needle type fangs when it bites.
When threatened, a rattlesnake will coil itself and rear its head about 6 in. above its body and rattle its tail. If it still feels threatened, it will strike at the enemy. A rattlesnake can strike up to half of its body length away, so a 4 foot long snake has a strike zone radius of about 2 feet.