By vaccination, immunity to small-pox is induced by the introduction of the virus of a disease which is either an allied condition or a weakened form of the disease against which protection is sought. Vaccination is the type after which many modern methods of producing immunity have been devised. Of late years another method has been introduced, in which antitoxines present in the blood of immunized animals are used to produce immunity in susceptible animals.

Vaccination as a mode of producing immunity implies the production of a modified form of the disease, and this is done chiefly by so treating the infective agents as, in the first place, to modify their intensity. Cultures of pathogenic microbes may be weakened in a number of different ways, such as by exposing them to a slightly higher or a lower temperature than suits their habit, as in the case of the anthrax bacillus; by cultivating for a prolonged time without passing the microbes through the body of an animal; by adding certain chemical substances, etc. By various modifications of the different methods, vaccines have been prepared of a number of diseases of which the principal are Fowl cholera, Anthrax, Hog erysipelas, Symptomatic anthrax, and Rabies.

An important step in advance was made by the discovery that in order to produce immunity by vaccination it is not necessary, at least in some cases, to introduce the microbes, but that their toxines are sufficient for the purpose. There is thus an immunity producible by chemical vaccines. The bacteria are separated from the cultures by filtration through unglazed porcelain, or they are killed by heat or by means of an antiseptic agent, and the toxines, thus freed from living microbes, are inoculated. By such means immunity has been procured in animals to malignant oedema, hog cholera, tetanus, diphtheria, and some other forms of infective disease. Wooldridge found not only that cultures of anthrax grown in certain special media produced immunity after the microbes had been separated, but also that immunity was procurable by means of substances which had no connection with anthrax, namely, certain forms of fibrinogen. As a further illustration of immunity by means of chemical agents, it may be stated that immunity to the poison of the cobra or of the rattle-snake may be obtained by careful inoculation with these poisons in increasing doses. The same applies to the vegetable poisons rkin obtained from the castor-oil bean, and abrin obtained from the jequirity bean.

Antitoxines

This word represents a further advance in the production of immunity. It has been found in the case of an ever-increasing number of microbes that the blood serum of an animal which has been rendered immune, when injected into a susceptible animal, renders the latter immune to that form of infection. Not only that, but the blood serum added to the toxines produced by artificial cultures renders these toxines innocuous when injected into animals. These facts,,which were established by Behring and Kitasato in the case of tetanus and diphtheria, imply that in the immunized animal a substance is produced which antagonizes the toxine, and is therefore called antitoxine. There have been established on this basis methods of treatment full of promise, to which the term Serum-therapy is given. The extraordinary activity of the antitoxine in controlling the deadly toxine of tetanus may be estimated from the following facts (Roux). Taking an exceedingly active tetanus toxine, there is added to it a quantity of serum from an immunized animal, equal to 1/900 part of the toxine solution. A half cubic centimetre of the mixture containing an otherwise fatal dose of the toxine is inoculated in a guinea-pig, but the animal remains unaffected. The activity of the antitoxine is indicated by the fact that the amount of serum used is only the one-thousandth part of a cubic centimetre.

The emciency of the antitoxines of tetanus and diphtheria, either when applied to the toxines directly or injected into the animal at the same time as the toxines, is unequivocal. In the case of the respective diseases the success of treatment will depend on whether the antitoxine is administered in time to prevent the fatal action of the toxine. Observations seem to show that this can seldom be done in tetanus, as the disease has no local manifestations, and the general symptoms which proclaim it already imply the presence of a general toxic action. In diphtheria, however, the local manifestations are distinct, and there is usually time, if these are recognized soon enough, to anticipate the general toxic effects by injection of the serum. Hence the so-called Serum-therapy is efficient in diphtheria but not in tetanus. In regard to other diseases little has yet been done, but there are evidences that in man the blood serum of those who have passed through an attack of pneumonia, typhoid fever, and cholera, confers immunity in animals to the microbes concerned in these diseases.