To the morphological difference of the phagocytes corresponds also a difference in the properties of their cytases, which are suited to the digestion of this or that food. The cytases are kept within the interior of the cells and only escape into the humors when the phagocytes are damaged (Pfeiffer’s phenomenon). This kind of ferment does not withstand a temperature above 55° to 58° C. In natural immunity, it plays the principal part by digesting morbid agents inside the phagocytes like any other food. But, in artificial immunity, other soluble ferments come into play, developed in consequence of vaccination.
The principal of those is the fixator.[26] It is less sensitive than cytasis to high temperatures and can bear a temperature of 65° to 68° C. It is incapable, by itself, of killing and digesting, but by fixing on them, it bites them, so to speak, and makes them sensitive to the action of the phagocytic cytases, which can thus digest them more easily.
The fixator may be compared to enterokinase, a special ferment in the small intestine of higher animals which also does not by itself digest food but which activates in a high degree the digestive power of pancreatic ferments. However, it has the property of fixing itself on fibrin; it is obvious that enterokinase and the fixator have the same essential properties. This similarity again proves that the destruction of morbid agents by the phagocytes really corresponds with actual digestion.
It is in consequence of the digestion of vaccinal products that the phagocytes manufacture the fixator. Created at the expense of a given vaccinal substance, the fixator has a specific character which corresponds with that substance, whereas the cytase already existing within the phagocytes never has a specific character.
Artificial immunisation generally produces the formation of so great a quantity of fixators that the phagocytes are unable to retain them and excrete them in part in the ambient humors, i.e. the blood plasma, or serum. When, afterwards, virulent morbid agents (microbes or figured elements) are introduced into an organism which has been immunised against them, they are at once faced, in the humors, with fixators, which immediately exert a biting action on them and render them sensitive to the action of the intracellular cytasis of the phagocytes. The same mechanism explains the specificity of the serums of vaccinated animals.
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.