To create some antivenoms, a little bit of venom is injected into an animal, like a horse. That animal's immune system will create antibodies which signal other cells in the immune system to destroy the venom. Then, those antibodies can be taken out of the animal's blood to make an antivenom. Then, when that antivenom is given to a person who has been poisoned by that venom, the person's immune system will already "know" to kill the venom.