duced immunity by injecting a culture of diphtheria bacilli in a broth of thymus gland, after having heated it at 65º to 70º C. (149º to 158º F.) for a quarter of an hour, they having assumed that the thymus extract exercised an antitoxic influence on the specific diphtheria toxine. Roux and Vaillard immunized animals by a mixture of three parts of toxine and one part of Gram's solution of iodine, the substances being mixed a few moments before they were injected beneath the animal's skin. Roux found that a rabbit of medium weight easily supported an injection of half a cubic centimetre of that liquid, and after a few days the injection could be renewed and so continued during a few weeks, when the injection would be increased in quantity or the pure toxine might be administered. He also found that it was necessary to frequently weigh the animals and to interrupt the injections when they lost weight, otherwise a depraved condition of the animal's system developed, that might terminate fatally. Animals thus immunized may be injected with a dose of toxine, or a quantity of culture of virulent bacillus that would ordinarily be fatal with but little if any unpleasant effect.
In 1890 Behring demonstrated the fact that blood-serum taken from an immunized animal was capable not only of producing immunity from the same infectious principle in another animal, but, further, that it possessed the power of curing an infection already in progress. This latter remedial employment of serum containing some antitoxine is called serum therapy. The serum is called an antitoxine serum because it contains some agent that antagonizes the toxine.
Besides the serum, Ehrlich, Roux, and others found that the milk of goats and cows that had been immunized was a source of antitoxine, though such milk was much less active than the serum.
The investigators found that of all the animals capable of furnishing large quantities of antidiphtheritic serum the horse was most easily immunized. Roux frequently found horses in which the injection of from two to five cubic centimetres of strong toxine beneath the skin provoked only a transient fever and a local swelling that quickly disappeared. The cow and the ass were found to be much more susceptible to the action of the toxine. Behring held that the antitoxic properties of the serum furnished by an immunized animal were greater in proportion to that animal's sensitiveness to the action of the toxine. But Roux did not consider this an established fact, and since 1892 had employed horses for immunization against diphtheria, because horse serum was not harmful when injected into lower animals or man, and from the jugular vein of a horse large quantities of blood might be obtained from which a perfectly clear serum could be separated.
