For instance, working in collaboration with MM. Roux and Salimbeni, he had found that it is by soluble poisons that the cholera vibrions harm the organism or kill it, but that small doses of the same _poisons_ are vaccines and make the blood of the vaccinated animal _antitoxic_. On the other hand, a _microbian_ vaccination is preventive against _microbes_ only but not against toxins and the blood does not become antitoxic. This is explained by the fact that it is not the same cells which digest cholera microbes and cholera toxins: the _microphages_ digest the vibriones whilst the _macrophages_ digest the poisons and, probably, manufacture as products of this digestion, the corresponding antibody, the cholera antitoxins.
On the contrary, in cases of the inclusion of _microbes_ by _macrophages_, as, for instance, in plague, the blood acquires an _antitoxic_ power by injection of the microbes themselves and not by their toxins, as was demonstrated by M. Roux and his collaborators. The same fact was observed by Metchnikoff on the alligator, in whom also microbes are digested by _macrophages_. In those cases, when microbes and toxins are digested by the same cells, the latter manufacture antibodies against both.
These facts rendered legitimate the supposition of the macrophagic origin of antitoxins.
In 1897 an International Congress took place in Moscow. Metchnikoff read a paper on the phagocytic reaction against toxins and another dealing with the whole of the knowledge acquired concerning human plague. He ended this by a plea in favour of Science, so often accused of having contributed nothing to the solution of the most important human problems, particularly ethical ones, and of having, on the contrary, sanctioned the law of Might by tabulating the laws of the struggle for existence. Metchnikoff objected that, far from doing so, Science, by revealing the laws of Nature, applied to humanity the benefits derived from them, whilst striving to counterbalance their cruel or harmful effects. The struggle against plague and other diseases was a concrete example of this, for here medical science opposed itself to the cruelty of "natural selection." He wound up his speech by the following words, "Just as, in order to satisfy his æsthetic tastes, Man revolts against the laws of Nature which creates races of sterile and fragile flowers, he does not hesitate to defend the weak against the laws of natural selection. Science has been faithful to her mission and to her generous traditions. Let her, then, progress unhindered."