With these facts in view, let us proceed to consider briefly the various theories which have been offered in explanation of acquired immunity:
Exhaustion Theory.—For a time Pasteur supported the view that during an attack of an infectious disease the pathogenic micro-organism, in its multiplication in the body of a susceptible animal, exhausts the supply of some substance necessary for its development, that this substance is not subsequently reproduced, and that consequently the same pathogenic germ can not again multiply in the body of the protected animal.
In discussing this theory, in a paper published in the American Journal of the Medical Sciences (April, 1881), the writer says:
Again, how shall we account for the fact that the amount of material which would nourish the small-pox germ, to the extent of producing a case of confluent small-pox, may be exhausted by the action of the attenuated virus (germ) introduced by vaccination? Pasteur's comparison of a fowl protected by inoculation with the microbe of fowl cholera, with a culture fluid in which the growth of a particular organism has exhausted the pabulum necessary for the development of additional organisms of the same kind, does not seem to me to be a just one, as in the latter case we have a limited supply of nutriment, while in the former we have new supplies constantly provided of the material—food—from which the whole body, including the hypothetical substance essential to the development of the disease-germ, was built up prior to the attack. Besides this, we have a constant provision for the elimination of effete and useless products.This hypothesis, then, requires the formation in the human body, and the retention up to a certain time, of a variety of materials which, so far as we can see, serve no purpose except to nourish the germs of various specific diseases, and which, having served this purpose, are not again formed in the same system, subjected to similar external conditions, and supplied with the same kind of nutriment.
It is unnecessary to discuss this hypothesis any further, inasmuch as it is no longer sustained by Pasteur or his pupils, and is evidently untenable.
The Retention Theory, proposed by Chauveau (1880), is subject to similar objections. According to this view, certain products formed during the development of a pathogenic micro-or-