There is only one class of physiological processes in which the type of the real secondary regulation occurs. The discoveries of the last twenty years have proved beyond all doubt, and future discoveries will probably prove even more conclusively, that the so-called immunity against diseases is but one case out of numerous biological phenomena in which there is an adaptive correspondence between abnormal chemical stimuli and active chemical reactions on the part of the organism and in its interior, exceeding by far everything that was formerly supposed to be possible in organic regulation.
The adaptive faculty of the organism against inorganic poisonous substances[110] is but small comparatively, and is almost always due not to a real process of active regulation but to the action of substances pre-existing in the organism—that is, to a sort of adaptiveness but not adaptation. Metallic poisons, for instance, may be transformed into harmless compounds by being combined with albumen or sulphuric acid and thus becoming insoluble, or free acids may be neutralised, and so on; but all these processes go on to a certain extent only, and, as was mentioned already, are almost always the result of reactions with pre-existing materials. Only in a few cases is there any sort of true adaptation to metallic substances, such as sublimate and, in a very small degree, arsenic, comparable in some respects with the adaptation to abnormally high temperatures. The organism which has been accustomed to receive at first very small amounts, say, of sublimate, and then receives greater and greater amounts of this substance by degrees, will at the end of this treatment be able to stand a quantity of the poison that would have been instantly fatal if administered at the first dose.[111] But the explanation of this adaptation is not known in any case; there seems to be some similarity between it and the so-called histogenetic immunity against organic poisons.
It is in the fight against animal and vegetable poisons, such as those produced by bacteria, by some plants and by poisonous snakes, that the true adaptation of the organism reaches its most astonishing degree. The production of so-called “anti-bodies” in the body fluids is not the only means applied against noxious chemical substances of this kind: the existence of so-called histogenetic immunity is beyond all doubt, and Metschnikoff[112] certainly was also right in stating that the cells of the organism themselves repel the attack of living bacteria. Cells of the connective tissue and the white blood cells, being attracted by them as well as by many other foreign bodies, take them in and kill them. This process, called “phagocytosis” is of special frequency among lower animals, but it also contributes to what is called inflammation in higher ones.[113] And there are still other kinds of defence against parasites, as for instance the horny or calcareous membranes, employed to isolate trichinae and some kinds of bacteria. But all this is of almost secondary importance as compared with the adaptive faculties of the warm-blooded vertebrates, which produce anti-poisonous substances in their lymph and blood.
It is impossible to say here[114] more than a few words about the phenomena and the theory of immunity proper, which have attained the dimensions of a separate science. Let me only mark those general points which are of the greatest theoretical interest. Discoveries of the most recent years have shown not only that against the “toxins” of bacteria, snakes, and some plants, the organism is able actively to produce so-called “anti-toxins”—that is, soluble substances which react with the toxins and destroy their poisonous character—whenever required, but that against any foreign body of the albumen group a specific reaction may occur, resulting in the coagulation of that body. But the destruction of the noxious substance or foreign albumen actually present is not all that is accomplished by the organism. “Acquired immunity” proper, that is, security against the noxious material for a more or less extensive period of the future, depends on something more. Not only is there produced as much of the so-called “anti-body” as is necessary to combine with the noxious, or at least foreign substances which are present, but more is produced than is necessary in the actual case. On this over-production depends all active immunity, whether natural or, as in some kinds of vaccination, artificial; and so-called “passive” immunity, obtained by the transfusion of the serum of an actively immune organism into another also depends upon this feature.[115]
This phenomenon in particular—the production of more of the antitoxin or the “precipitin” than is actually necessary—seems to render almost impossible any merely chemical theory of these facts. The reaction between toxin and antitoxin, albumen and precipitin is indeed chemical; it may in fact be carried out in a test-tube; but whether the production of the anti-body itself is also chemical or not could hardly be ascertained without a careful and unbiassed analysis. There can be no doubt that the well-known theory of Ehrlich,[116] the so-called theory of side-chains (“Seitenkettentheorie”) has given a great impulse to the progress of science; but even this theory, irrespective of its admissibility in general, is not a real chemical one: the concept of a regeneration of its so-called haptophore groups is a strictly biological concept.[117]
And, indeed, here if anywhere we have the biological phenomenon of adaptation in its clearest form. There are very abnormal changes of the functional state of the organism, and the organism is able to compensate these changes in their minutest detail in almost any case. The problem of the specification of the reactions leading to immunity seems to me, as far as I can judge as an outsider, to stand at present in the very forefront of the science. There cannot be the slightest doubt that especially against all sorts of foreign albumens the reaction is as strictly specific as possible; but there are some typical cases of specificity in the production of antitoxins also. It is, of course, the fact of specific correspondence between stimulus and reaction, that gives to immunity its central position among all adaptations, no matter whether the old hypothesis of the production of specific anti-bodies proves tenable, or whether, as has been urged more recently by some authors, the anti-body is always the same but reacts differently according to the medium. In the latter case it would be the medium that is regulated in some way by the organism in order to attain a specific adaptedness.
NO GENERAL POSITIVE RESULT FROM THIS CHAPTER
But now let us look back to the sum of all the physiological reactions studied, and let us see if we have gained a new proof of the autonomy of life from our long chapter.
We freely admit we have not gained any really new proof, but we may claim, I think, to have gained many indicia for the statement that the organism is not of the type of a machine, in which every single regulation is to be regarded as properly prepared and outlined.