antibody, formed in the body by the introduction of a bacterial poison, or toxin, and capable of neutralizing the toxin. People who have recovered from bacterial illnesses often develop specific antitoxins that confer immunity against recurrence.

For medical use in treating human infectious diseases, antitoxins are produced by injecting an animal with toxin; the animal, most commonly a horse, is given repeated small doses of toxin until a high concentration of the antitoxin builds up in the blood. The resulting highly concentrated preparation of antitoxins is called an antiserum.

The first antitoxin, to diphtheria, was discovered in 1890 by Emil von Behring and Shibasaburo Kitasato (Kitasato Shibasaburo), for which Behring received the 1901 Nobel Prize for Physiology or Medicine. Today, antitoxins are used in the treatment of botulism, diphtheria, dysentery, gas gangrene, and tetanus. If the toxin is a venom, the antitoxin formed, or the antiserum containing it, is called an antivenin. See also antiserum.