Have you ever gotten a vaccination? When you get vaccinated for say, Hepatitis A, you actually are injected with a very small amount of the virus so that your body can build up the correct immune response to that specific virus. It´s the same with the flu shot - flu shots give you a small and controlled dose of the flu that your body is able to recognize and attack so that if the actual flu comes around you have all the virus specific antibodies necessary to attack it. Well it is exactly the same with venom. The only way the body can appropriately fight against the venom is by recognizing the venom and knowing which antibodies to employ to fight against it. Unfortunately, even a very small amount of venom may be deadly to a person, so instead, we use other animals to produce the antibodies for us. Horses are commonly injected with a very small, non-lethal amount of snake´s venom. The horse´s immune system then responds and begins to attack the venom, producing antibodies that carry out this attack in the process. Anti-venom is actually the harvested antibodies. Rather than waiting for our body to produce antibodies (which we may not have time for in the case of very deadly venom) we just give our body the antibodies it needs in the form of anti-venom.