An antitoxin is a preparation of antibodies against a toxin such as diphtheria or botulism or against animal venoms such as black widow spider venom (or the one for Australian funnel web spiders, for you blokes down under - G'day 8-) or various snake venoms.

Antitoxins are produced by injecting the harmful toxin into an animal such as a horse in gradually increasing amounts. If the doses are correctly chosen, this does not harm the animal at all. The animal then produces its own antibodies to the toxin. Blood is then withdrawn from the animal, and these antibodies are extracted from the blood and purified. When injected into a human patient, the antibodies (the antitoxin) selectively attack and neutralize the toxin present within the patient's system.

Antitoxins are not used very often in clinical medicine for two reasons. First is that they are only useful against certain diseases in which the toxin is largely responsible for the worst effects of the disease. More importantly, perhaps, is the fact that injecting foreign animal proteins into the human bloodstream carries certain risks, notably an immunologic phenomenon known as serum sickness that occurs several weeks after the antitoxin injection.

This reaction is potentially serious enough to warrant careful consideration of the potential risks versus benefits of the treatment. Eventually, we can predict that genetically engineered human antitoxins (monoclonal antibodies) will be used instead of the cruder and side-effect prone animal products.

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