How is antivenin made?

First they milk a venomous snake, then they take a very, very small amount of the venom and inject it into a horse, goat, sheep, or cow, and gradually, over a period of time slightly increase the amount of venom injected. The animal will develop a tolerance to the venom. Then antibodies are extracted from the blood of the animal and this is basically what comprises the antivenin.

Madsnakeman's absolutely correct, only thing I'd add is that there are also a few varieties of antivenin being made with egg albumen. Horse antivenin has an issue of people being allergic to the protiens in it... sheep antivenin tends to not cause so many bad reactions, and as far as I'm aware negative reactions to albumin-based antivenins are nearly nonexistant, the problem is, the smaller the animal you're making antivenin with, the smaller the venom doses the animal can recieve... I believe there are some venoms that horse antivenin is still the only thing going for.