The quantity of specific fixators in the humors depends on the surplus production of that ferment by the phagocytes and is not always the same. That is why different serums are preventive in different degrees. They are inactive if the phagocytes have not produced enough fixators to pass any out into the humors. For a serum is only preventive when it brings into the new organism into which it is injected a sufficient quantity of fixators ready to sensibilise the morbid agents afterwards introduced into the organism.
The over-production of antibodies--fixators or antitoxins--corresponds up to a certain point with the frequency and quantity of vaccinal injections; that is why serums are usually preventive in artificial immunity and very rarely so in natural immunity. Through successive inoculations, the cells become accustomed to digesting the microbes, or figured elements, and manufacture, in consequence of that digestion, growing quantities of fixators.
In natural conditions, on the other hand, morbid agents do not usually penetrate into the organism in massive or repeated doses; therefore digestion under natural conditions results in a less abundant production of fixators which can be contained in the interior of the phagocytes without leaking into the humors in sufficient quantities to render the latter preventive.
It might be thought that immunity against pathogenic microbes is accompanied by immunity against their toxins. In reality that is not always the case, and very often the organism, now made refractory to certain microbes, remains sensitive to their toxic products. Thus antimicrobian immunity and antitoxic immunity constitute in most cases two distinct properties. In order to confer antitoxic immunity recourse must be had to vaccination by soluble poisons and toxins.
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.