As has been stated, if small doses of some special poison, such as diphtheria toxin, be repeatedly injected into a susceptible animal in increasing amount there will be developed in the blood-serum of that animal an antibody, called an antitoxin. This is formed by the cells and, according to Ehrlich's side-chain theory, corresponds to the free receptors. By injecting the antitoxin into an immunized animal it can resist a dose of toxin that ordinarily would be several times more than necessary to kill. That a combination occurs between the toxin and antitoxin can be proved by mixing the two together in a test-tube. The resulting mixture will prove harmless when injected into a susceptible animal.

Antitoxins are destroyed by heat, acids, and many chemicals, and gradually deteriorate spontaneously when in solution, particularly when kept at room-temperature. To preserve their activity the temperature should be not more than 50 C. Antitoxins are specific in that they neutralize the corresponding toxin and have no other apparent action within the body. The occasional ill effects, such as serum sickness, following the injection of antitoxic serums are due to other substances (the proteins in the serum) and not to the antitoxins themselves. Antitoxins may be injected subcutaneously, intravenously, into the subarachnoid space, into a nerve, into the brain substance, or into any of the body cavities. They are practically useless when given by the mouth, as very little is absorbed. As antitoxins, when injected into an organism, tend to disappear rather quickly, passive or antitoxic immunity is, therefore, transient; it cannot be depended upon for more than ten days or two weeks. When antitoxic serum is injected subcutaneously the antitoxin is absorbed slowly, requiring about forty-eight hours before it appears in the blood in maximum amount. When very prompt action is necessary the antitoxin should be introduced directly into the circulation by intravenous injection. Antitoxins are valuable both as curative and immunizing agents. For curative purposes they should be given early and in large enough doses to get their action before damage has been done by the toxins.