Immunity, acquired naturally, is so especially against microbes and not against toxins, for, in nature, it is almost always by microbes that the organism is threatened. As to _antitoxic_ immunity, it is very probably due to the intracellular digestion of toxins by the different macrophages. This hypothesis is supported by the experiments quoted in the preceding chapter. During antitoxic vaccination, the macrophages manufacture, probably at the expense of vaccinal toxins, a certain quantity of _antitoxins_, substances which offer a great similarity with the fixators. Like them, they are specific; they are also produced in great quantities and excreted into the humors, which they render antitoxic when sufficiently abundant; finally, they are not very sensitive to high temperatures. That is why, in spite of the impossibility of proving their origin directly, it is quite probable that it is analogous to that of the fixators and that antitoxins are manufactured by cellular elements, the macrophages in particular. For it is they which absorb and digest toxins as well as soluble poisons. This deduction is also supported by the antitoxic immunity which may be conferred on _unicellular_ beings in which the cell alone enters into play. Phagocytes no doubt manufacture many other soluble ferments corresponding with the elements which they absorb, for, in a vaccinated organism, divers new specific properties of the serum are to be found, such as that of agglutination, precipitation, etc. Humoral properties may be more or less durable, in proportion as the products manufactured by the phagocytes are more or less rapidly evacuated by the organism. All these humoral properties, traced back to their first source, depend upon the digestive activity of the phagocytes, since they are the products of that digestion. In cases where it has not yet been possible to make a direct demonstration of this, it becomes evident through analogy and experiments pointing in that direction. To sum up, according to Metchnikoff, "_Immunity in infectious diseases is linked with cellular physiology, namely, with the phenomenon of the resorption of morbid agents through intracellular digestion._ In a final analysis, the latter (as also the digestion of food in the gastro-intestinal tube) reduces itself to phenomena of a physico-chemical order; however, it is a real _digestion_ accomplished by the living cell.... The study of Immunity, from a general point of view, belongs to the subject of Digestion."