Antivenoms are purified antibodies made by mammals such as horses, sheep, rabbits and dogs. Their immune system makes these molecules which 'recognize' the molecules in the venom and attach to them. Even after the venom is out of its system, the animal has high levels of antibodies in its bloodstream. When the antibodies are injected into the bloodstream of a human who has been bitten or stung, they attach to the molecules in the venom, rendering them harmless.