IMMUNITY. It has been a common ob servation that certain individuals are prone to the infectious diseases, and others not. Some children in a family always "take things hard,* while others are not susceptible. This resist ance to the poisons of the infectious diseases, or to the bacteria that cause the poisoning, is termed immunity. It is a relative quality of living matter, and is not confined to man alone, for lower animals, and even plants, show marked variations in their reactions to chem ical and organic poisons. Thus certain plants are capable of growing in soils impregnated with metallic poisons of sufficient strength to destroy animal life; many animals are immune to poisons that would kill man; thus birds are comparatively immune to strychnine; young children can take comparatively larger doses of belladonna than adults; the diseases of plants are almost unknown among animals, and vice versa ; certain diseases affect the lower animals and are rare or unknown in humans, and the reverse condition also holds true; thus swine plague is comparatively unknown in other ani mals than pigs, and such human diseases as typhoid fever and cholera are not common in lower animals; thus plants, lower animals and man, individually and collectively, enjoy certain relative immunity from destruction when ex posed, under ordinary circumstances, to disease producing agencies.
Natural immunity may be so modified as to be lost entirely, certain forms of disease pre disposing the sufferer to ready secondary in fection, as, for instance, is seen when tubercu losis follows measles; or a partial immunity be made more effective, or a new immunity con ferred. This has been termed acquired im munity. The evolutionary doctrine would tend to interpret natural immunity as an inherited acquired immunity. In the case of man im munity sums up those powers of resistance which the body naturally possesses, or which it acquires in the struggle with infectious dis eases, both in endeavoring to destroy the bac teria —bacteriolytic power — and to counter act the toxics — antitoxic power. Modern pathology has shown that the battle-field is a large one, and that the opposing forces are nu merous and their powers largely unknown; and it must be remembered that the struggle has been going on for millions of years.
The observation that certain diseases — as measles, scarlet fever, smallpox— once ac quired, confer a marked immunity, led up to vaccination, this inoculation against smallpox being the first conquest of disease by such means. It is certain that others will follow. Certain diseases — notably pneumonia, influ enza, erysipelas—confer an immunity, but it is not lasting; thus demonstrating the principles of variability in the "immune bodies" as a class, and of an active and a passive immunity which may be conferred by various means, an active immunity being acquired by the animal for it self by direct adaptation, a passive immunity being conferred by a body made in the blood serum of another animal.
Former theories for explaining the varied picture in this rapidly widening study have been numerous. They may be classed under the exhaustion theory of Pasteur, which assumed that the bacteria used up the available food supply and died; or, as the laity often express it, "the disease wore itself out This theory has been thoroughly disproved. The retention heory —that the bacteria are killed by their own products—is also untenable. The me chanical, humoral and p hag ocytosis theories, which teach that the bacteria are destroyed by the humors or cells of the body, are partly true, but do not convey the whole truth, which in fact may never be known. The most popu lar theories of the present time are chemical, and that known as Ehrlich's theory is uppermost in discussion. This theory is ex tremely elaborate, but its fundamental principle is that the blood-serum of man and other ani mals may be so modified, in whole or in part experimentally played upon, as it were— that it can be made to overcome the effects of infec tions, of poisons, or of both. The development of the diphtheria antitoxin in the blood-serum of the horse, to counteract the effects of the toxin of the diphtheria bacillus in man, was the first important practical deduction of this great principle. It was the first illustration of the production of a successful passive immunity in human pathology. In the discovery of the diphtheria antitoxin it was hoped that all the infectious diseases were conquered, but this hope was premature, as it was learned that other diseases involved other factcirs of a more elusive character than the simple toxin. The destruction not only of the bacteria within the body, but the neutralization of the poison as well, was found necessary, and hence the terms bacteriolytic immunity and antitoxic immunity. Ehrlich's side-chain theory tries to explain anti toxic immunity in chemical terms. He assumes that the cell-body has a number of side-chains upon it—receptors, as they are termed. These are capable of combining with food-products for the metabolism of the cell. There are cer tain receptors that can combine with toxic products as well, with damage to the cell. Anti toxins, according to Ehrlich, consist of surplus receptors made by the cell and cast off in the blood-serum. These unite with the toxin in the serum, and thus save the receptors of the cell for their normal food-taking properties. Any surplus of receptors over and above those com bined with the toxin molecules floating in the blood are available as free antitoxins in the treatment of toxin-caused disease.