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the tetanus toxin. But the usual source from which antitoxins are obtained for practical purposes is the blood of animals which have been rendered immune. The diphtheria antitoxin, which is now extensively and successfully employed for the cure of diphtheria, is obtained from the blood of horses which have been immunised by repeated inoculations with the toxic products of the diphtheria bacillus.

A most interesting question presents itself in connection with the discovery of the antitoxins : Is the animal which has been immunised against any particular toxin also immune for other poisonous substances of the same class ? This question has been definitely answered in the negative by experimental investigation. In other words, each specific toxin causes the development in the body of the immune animal of a specific antitoxin which has no neutralising action upon any other toxin than that which gave rise to its production. An animal which has been immunised against the toxic action of ricin is poisoned by the usual fatal dose of abrin. The tetanus antitoxin affords no protection against the poisonous products of the diphtheria bacillus and vice versa.

The antitoxins protect susceptible animals from infection when introduced by inoculation at the same time or in advance of the disease germs against which they have a specific action. They may also, in

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some cases, be successfully used in the cure of infectious diseases, when these have not advanced too far.

The remarkable success attending the use of the diphtheria antitoxin for this purpose is well known and a certain degree of success has attended the efforts of physicians in the treatment of other dis. eases by the same method-tetanus, erysipelas, pneu: monia. But specific treatment by antitoxins is still in its infancy and much careful experimental work and clinical experience will be necessary in order to determine the practical value of this method in the diseases mentioned and in other infectious maladies. Enough is known at present, however, to lead to the hope that when methods have been devised for obtaining these various antitoxins in a pure and concentrated form they will constitute a most valuable addition to our resources for the treatment of infectious diseases. Indeed the only hope of specific medication for such diseases appears to lie in this direction.

In the present volume I shall not attempt to discuss the questions connected with the origin of the antitoxins in the bodies of immunised animals, the chemical nature of these substances, or the mode of their action in neutralising the toxins. These are questions which would involve a considerable amount of technical knowledge on the part of the reader for an understanding of the most advanced views regarding them, as expounded by Ehrlich and others—Ehrlich's “side chain theory.” In a popular treatise a simple statement of well ascertained facts will, I hope, be appreciated, while an exposition of theories still under discussion might prove wearisome.

It has been shown that the antitoxins when mixed with toxins in a test-tube exhibit their specific neutralising action as shown by the innocuousness of the mixture when injected beneath the skin of a susceptible animal.

The antitoxin of snake poison, which has been successfully used in India for the cure of persons bitten by the deadly cobra, when mixed with cobra venom in proper proportion completely neutralises the poisonous properties of this venom. Such a mixture injected beneath the skin of a small animal is without effect. But if the mixture is heated to 70° C. the antitoxin is destroyed and by inoculation experiments the toxin is found to be still present and active.

The facts stated show that in certain infectious diseases acquired immunity depends upon the formation of antitoxins in the bodies of immune animals. But these antitoxins have no power to destroy specific disease germs. They neutralise the toxic products of these germs without exhibiting any germicidal action upon the germs themselves. As,

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however, the power of the germs to overcome the resources of nature and invade the blood or tissues depends upon the toxic products developed by them, they are deprived of their power to multiply in the bodies of living animals when these poisonous substances are neutralised. Practically they become as harmless as the common “saprophytic bacteria” which surround us on all sides, and are swallowed in countless numbers with every glass of unsterilised water we drink.

But there is another class of substances, developed during certain diseases, which exhibit specific germicidal activity and have no antitoxic value. Such substances are found in the blood of animals which have been made immune to the pathogenic action of the cholera spirillum, the typhoid bacillus, and the bacillus of hog cholera.

The writer, some ten years ago, obtained experimental evidence which indicates that smallpox immunity probably depends upon a substance which destroys the smallpox germ, rather than upon an antitoxin.

Further details with reference to the antitoxins will be found in Part Second of this volume, in which questions relating to infection, disinfection, and immunity will be discussed in connection with the more important infectious diseases, considered separately.

PART SECOND

SPECIFIC INFECTIOUS DISEASES

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