These observations suggested to Metchnikoff the idea that senile atrophy might be due to the same mechanism, and his thoughts turned towards the problem of the causes of old age.
But, before undertaking researches in a new direction, he wished to conclude those he had been pursuing for twenty years on the phenomenon of phagocytosis. He therefore started to complete his investigations on immunity in order to epitomise them and to give a definite form to his doctrine on that subject.
[CHAPTER XXVI]
1900. Immunity — Natural Immunity — Artificial Immunity.
For centuries the question of immunity has occupied the human mind because the prevention of disease has ever been one of the greatest preoccupations of Man. Savages had already observed that man can become refractory to the venom of serpents, either through a slight bite or by the application of certain preparations of that venom on scarified skin. It was also a popular and very ancient notion that the contact of a slightly scratched hand with the pustules of cow-pox conferred immunity against human small-pox. It was on this observation that Jenner founded his method of antivariolic vaccination. The latter, in its turn, suggested to Pasteur the idea of attempting antimicrobian vaccinations. Having ascertained that old cultures of chicken cholera, previously very virulent, had become harmless, he wondered whether they had become a vaccine and proved by experiment that they had. That led him to the principle of the attenuation of viruses and to that of vaccination by attenuated microbes. Thus the problem of the mechanism of immunity was stated.
The first theories propounded on the subject concerned the humors. Pasteur supposed that immunity was due to the absorption, by the vaccinating microbes, of certain nutritive substances in the humors, which, not being renewed for some time, were missed by the microbes afterwards introduced into the organism, which therefore could not develop completely. Chauveau, on the other hand, thought that, in cases of immunity, the humors contained substances which were unfavourable to microbes. Those theories explained particular facts, but were not applicable to the generality of cases.
Other theories,[21] whilst attributing an active part to the organism itself, failed to account for the mechanism of immunity in general. This was due to the fact that knowledge at that time lacked the two essential elements, i.e. the modifications suffered by the organism which was becoming immunised, and the fate of the microbes in the refractory organism.
The disappearance of the microbes in the cured or refractory animal had indeed been observed;[22] the inflammatory reaction of the organism in the course of immunisation had been noted;[23] microbes had long ago been observed inside the white globules of pus;[24] but, either an erroneous interpretation was given to the facts observed, or, rather, the links of causality between those factors failed to be established because they were observed solely in the complicated organism of superior beings. Humoral theories, less easy to test, preserved an appearance of generality and were easily admitted.