Immunity in a person or animal must, there fore, in most cases, mean two things. It must certainly mean, in the first place, the tissues and blood have been made somehow in capable of affording the material for the growth and multiplication of the organism; for if the organism could grow, its toxins would be pro duced. And in the second place, it may mean that the toxins, though they gain entrance to the blood, are somehow antagonized, so that they produce no harm.
Now experiment has proved that both these results can be artificially produced in men and animals.
Of course, before this can be done for any particular disease, it is necessary that the organism which produces that disease should be known and be capable of artificial cultiva tion.
It has been shown, practically of all such organisms, -that they can be cultivated under such conditions as will either diminish or in crease their virulence, as may be desired. Thus varying strengths of the organism can be reared, and, beginning with a weak strain, they are injected, at proper intervals of gradually increasing strengths, into the animal or person to be made immune. Just as a mild attack of scarlet fever protects, at least for a time, from a virulent attack, so the weak organism protects the body from the subsequent stronger dose ; each succeeding injection, therefore, confers a greater immunity, until complete insuscepti bility is produced.
In the same way the organism may be reared in broth, and then the broth filtered. It con tains in solution not the organisms but the toxins. These also may be made weak or strong, and then repeated injections, beginning with small doses or doses of weak strength, gradually render the person or animal insuscep tible to the toxin. This method is successful not only against diphtheria and tetanus, but against snake poisons.
An animal or person made insusceptible in her of these two ways is said to be actively 'mune, that is, they resist an attack of the ;ease by reason of the quality imparted by e injections to their own tissues and blood.
Antitoxic in the course of the my observations and experiments made on is subject, a still more remarkable fact became apparent, the fact, namely, that the blood of one animal, made insusceptible in one or other of these ways to a particular infection, could pro tect another animal from attack, into whose body it was injected. This second animal is said to be passively immune, that is, it is not insusceptible on account of the resistance of its own blood and tissues, but in virtue of the im munized blood of another animal injected into it. In such a case it is not the whole blood that is used. Some blood is withdrawn from the body of the animal made actively immune, and under precautions allowed to stand. The blood separates by coagulation into clot and serum (see p. 294). The serum is drawn off,
and, to preserve it, mixed with a small propor tion of carbolic acid or such preserving agent, and then kept in sealed bottles. This serum, injected into another animal or into a person's body, will render that animal or person in susceptible to the attack of the particular infec tion. There are now highly scientific and accurate methods of estimating and regulating the strength of such sera, so that the amount required to be injected in any particular case can be judged. The animal whose blood is made the medium of preparing such sera is none the worse of the process. Indeed, if it were not kept in a condition of excellent health it would not be suitable for the purpose, and the amount of blood withdrawn from it at intervals is so regulated as to do it no injury.
The sera so prepared at present in use are the serum against diphtheria, that against tetanus, that against blood-poisoning (whose organism is the streptococcus), that against hydrophobia (rabies), that against plague, that against snake poison; or the and all classed as antitoxic sera. It is to be noted that these are neither prepara tions of the organisms nor of their poisons, they consist of the fluid of the blood of an animal immune to the organisms' attack. In the case of the anti-diphtheritic serum the animal used is the horse.
One great distinction must be noticed be tween the active immunity gained by injecting gradually increasing strengths of a culture of the organisms of a disease into a person's body, or a solution of the toxins, and the passive immunity gained by injecting the serum of a highly immunized animal; this, namely, that the first process takes some time, while the second process is practically immediate. If a person had already been infected by a disease, there would not be time to make him insus ceptible by the former method before the dis ease developed, while the use of a serum has immediate effect in preventing the develop ment of the disease. The antitoxic sera are, therefore, curative agents, if used soon enough after infection, while the methods of producing active immunity are only preventive. Of the latter method Hafl'kine's inoculation against cholera, and Wright's against typhoid, are excellent illustrations, while vaccination illus trates a method both preventive and, if used early enough—within four days of infection,— also curative.
There is evidence that the blood of a human being recovered from typhoid fever or cholera can confer immunity, and not the blood only, but some of the secretions, such as milk, and there are reasons for believing that in this way the milk of a nursing mother may render her child insusceptible to the infectious disease from which the mother has suffered.