Chapter XVI: Historical Sketch of Our Knowledge on Immunity (2)

Another Munich bacteriologist, H. Buchner, at first expressed himself[886] very favourably on the theory of phagocytosis. He regarded it as more capable of explaining most of the phenomena of immunity than was his own older local theory. But little by little he declared himself in formal opposition to the cellular theory of immunity and went over to the camp of his sometime adversaries. He adopted[887] the humoral theory of the bactericidal action of the body fluids, upon which subject he carried out several important investigations. He was able without difficulty to confirm Nuttall’s discovery of the disappearance of the microbicidal power when the defibrinated blood was heated to 55° C., and he added to this fundamental fact many others of great value. He demonstrated the part played by the salts in the exercise of this bactericidal power, and laid great stress on the fact that this power depends on the presence of a special substance of albuminoid nature, to which he gave the name of _alexin_. Buchner[888] combatted with success the idea that I had expressed, according to which the bactericidal power of the body fluids is reduced in great part to a plasmolytic action of the blood serum upon certain micro-organisms. It cannot be denied that my hypothesis is only very partially applicable, and that the larger share in the bactericidal action of the body fluids belongs to the alexins. Buchner also made the study of this action more easy by the demonstration that the red blood corpuscles of a foreign species undergo, under the action of the blood and of the serums, a globulicidal action comparable to that which occurs in the case of micro-organisms.

Whilst Flügge, von Behring and many others of the old partisans of the bactericidal theory of the body fluids abandoned it more or less completely as an explanation of immunity, Buchner remained faithful to it and tried, aided by the collaboration of his pupils, as far as possible to defend it.

[Sidenote: [552]]

In France this humoral theory was adopted chiefly by Bouchard[889] and his pupils, amongst whom I must cite more particularly Charrin and Roger. They sought to confirm it by personal researches, the greater part of which were carried out upon the bacillus of blue pus. These investigators studied it especially in relation to acquired immunity. A comparison of the mode of development of the pyocyanic bacillus in the serum of susceptible animals and of vaccinated animals of the same species, convinced them of the great importance of the action of the body fluids. In cases where these fluids were found to be incapable of killing the micro-organisms they exerted over them an injurious influence, either by attenuating their virulence, or by producing more or less important modifications in their forms and functions. The essential cause of natural or acquired immunity was always attributed by Bouchard’s school to the property of the body fluids. The phagocytes were said to intervene only secondarily, either to carry off the dead bodies of the micro-organisms, or to ingest the bacteria, rendered inoffensive by the humoral action.

The humoral theory of immunity, with some slight modifications, spread very generally into every country, and many investigators accepted it without reserve. But certain observers ventured to run counter to the general current and raised objections of principle against the theory of the bactericidal power of the fluids of the body. After the principal facts established by the partisans of this theory had been confirmed, it was asked whether the phenomena of the destruction of micro-organisms observed _in vitro_ are really equivalent to those produced in the refractory animal. A glance at the data brought together with so much zeal was sufficient to demonstrate that this parallelism does not exist. The blood of animals susceptible to certain micro-organisms was found to be bactericidal for these organisms, whilst that of refractory animals was incapable of destroying them. It is useless to cite examples, so numerous are they. On the other hand, the bactericidal power of the body fluids, so marked for certain pathogenic organisms such as the anthrax bacillus and especially the cholera vibrio and the typhoid coccobacillus, is insignificant or _nil_ as regards many bacteria against which refractory animals are not wanting.

[Sidenote: [553]]

All these facts throw doubt on the predominating part played in immunity by the bactericidal power of the body fluids. Lubarsch[890] attacked the humoral theory, showing by a great number of experiments that animals whose fluids are very bactericidal _in vitro_ are very susceptible to a much smaller quantity of bacteria of the same species introduced into the body. Thus, the defibrinated blood and the blood serum of rabbits destroy a large number of bacteria in a very short time, whilst the rabbits themselves contract fatal anthrax after the introduction of a small number of these micro-organisms into the blood vessels. This contradiction cannot be explained except by the profound changes which the blood must undergo outside the body. Facts of the same nature have been shown for the anthrax of rats by Hankin, Roux, and ourselves, as described in Chapter VI.

[Sidenote: [554]]

The International Congress of Medicine, assembled at Berlin in 1890, was the first occasion on which I spoke publicly of the new theories of immunity. In the addresses given at the general meetings, leaders of medical science in several countries summed up their opinion on this question. Koch[891], in his memorable report, declared that the new acquisitions had destroyed the basis of the theory of phagocytes, and that consequently it must give place to the humoral theory of immunity. Bouchard took up a more conciliatory position, but, according to him, the bactericidal power of the fluids of the body was the primary and essential cause of immunity. The phagocytes only intervened later, in order to finish the work begun without their assistance. Lord Lister expressed himself[892], on the other hand, much more favourably on the subject of the theory of phagocytosis. This observer, who is not only a great surgeon, but is perhaps even more remarkable for his great powers of generalisation, has paid special attention to the problem of immunity. With the object of clearing up this very complicated and at the same time important question, Lord Lister seized the occasion of the meeting of the International Congress of Hygiene in London in 1891, to bring about an exchange of views between the partisans of the various theories of immunity. Under his presidency he devoted an entire sitting of the Section of Bacteriology to the discussion of this question. Buchner presented a report[893] drawn up exclusively from the point of view of the humoral theory and devoted to the demonstration of the slight importance of phagocytosis, and also to the preponderant part played by the alexins dissolved in the body fluids and circulating in the plasma of the blood. He attempted to harmonise the facts on the bactericidal power of serums observed _in vitro_ with the special conditions to be met with in the animal body. He specially insisted on the point that, in the blood and the organs, the alexins cannot act with the same rapidity that they can in test tubes containing serum. In this way he recognised that between the bactericidal action _in vitro_ and that in the body of the animal, there exists a marked difference, but he would not consent to attribute it in the latter case to the intervention of the phagocytes.

Roux[894] also made a report on immunity at the same sederunt, speaking very distinctly in favour of the cellular theory. A chemist by inclination, he was sympathetic at first to the humoral theories of immunity. Working with Pasteur, and side by side with him, Roux, from the beginning of the new era of medical science, had made numerous experiments on the part played by the body fluids in immunity. But as the results were not sufficiently precise and demonstrative they were soon abandoned. The attachment of Roux, however, to the humoral theories was manifested in his work, carried out in part with Chamberland[895], on the subject of vaccination by means of microbial products. Later, having obtained a deeper knowledge of various facts concerning natural and acquired immunity, he rallied to the cellular conception and developed it in his report presented to the above Congress in London. Several microbiologists took part in the discussion, and I myself[896] was able to communicate certain facts concerning the immunity of guinea-pigs, acquired as the result of vaccination against Gamaleia’s vibrio. I chose this example because it presented, according to von Behring and Nissen, the clearest case of a bactericidal property developed during the course of immunisation. I was able to furnish the proof that, in the vaccinated animal, the micro-organism in question, in spite of the great bactericidal power of the blood serum _in vitro_, remains alive in the animal body for a long time, and that its destruction is effected by the phagocytes, which ingested it alive. In this example I showed that the leucocytes of the exudation, that have ingested vibrios, may still furnish cultures of this organism if they are taken from the body and transferred in hanging drop to the incubator.

[Sidenote: [555]]

The fact that, even in the case which appeared most to favour the humoral conception of acquired immunity, phagocytes play the principal part, must to many members of the Congress have appeared sufficiently significant. Indeed, several observers who were present at the debates, received the impression that the phagocytic theory had not been overturned by its adversaries. At this period the question of the importance of antitoxins from the point of view of immunity had scarcely been raised. The great discovery made by von Behring and Kitasato was already accepted by everyone; but there was no ground for attributing to it any general importance. In fact, though proved for tetanus and diphtheria, and extended by Ehrlich’s beautiful experiments to the vegetable toxins (ricin, abrin, and robin), the antitoxic property of the fluids of the body presented itself rather as a special than as a general phenomenon. It is in this sense that Roux had assigned to it its place in the chapter of immunity. The two diseases, against which antitoxic serums had been discovered, are certainly distinguished from the great majority of infections by the localisation of the micro-organisms and the abundant secretion of their toxins.

It was only after the London Congress that this question came prominently forward. Von Behring thought that the antitoxic power of the body fluids is generally distributed in all cases of acquired immunity, and that micro-organisms, introduced into the animal possessing this power, become incapable of any pathogenic manifestation. Certain facts, brought together in Bouchard’s laboratory, tell against the hypothesis I have just mentioned. With the object of throwing light on this question I began, immediately after the close of the Congress, to study the acquired immunity of rabbits against the micro-organism of the pneumo-enteritis of pigs. I was able to demonstrate[897] that in this case the resistance of the animal against the micro-organisms does not depend on the acquisition of any antitoxic property by the body fluids; such a property is completely absent. At the same time I showed that the serum of vaccinated rabbits possesses a very marked protective power against infection by the coccobacillus of pneumo-enteritis. It was for the first time proved that independently of the antitoxic and bactericidal properties of serums, there exists another special property, the anti-infective property. This I conceived to be of the nature of a stimulant action on the part of the phagocytes.

[Sidenote: [556]]

It has already been stated in an earlier chapter that before the discovery of antitoxins Richet and Héricourt[898] had observed an immunising action of the serum of animals refractory to staphylococci. These observers were content with this demonstration and did not seek to penetrate more deeply into the mechanism of the action of their serum. For this reason when von Behring and Kitasato announced their discovery of antitoxic serums it was generally thought that the antistaphylococci serums were also antitoxic serums. The immunity against the micro-organism of the pneumo-enteritis of pigs taught us that here we might have to deal with quite a different matter. It was soon demonstrated that the serum from the immunised animal might in fact, without being antitoxic, present the same anti-infective property as in the case of pneumo-enteritis. That was first proved in the case of the experimental disease set up by Koch’s cholera vibrio.

[Sidenote: [557]]

The reappearance of cholera in Europe in 1892 drew the attention of bacteriologists to this disease, and was the occasion of many new researches on immunity against the cholera vibrio. Several important works on this question were published by Pfeiffer[899], at this period director of the scientific staff of the Koch Institute at Berlin. He obtained, in animals well immunised against the cholera vibrio, a serum endowed with a high anti-infective power but entirely without any antitoxic property. The guinea-pigs themselves, very resistant to the cholera peritonitis, were found, on the other hand, to be very susceptible to the minimum lethal dose of the cholera poison. The absence of antitoxic power in the fluids of the body taken in connection with a well-marked phagocytic reaction in a large number of cases of immunity, natural and acquired, has turned the scale in favour of the cellular theory. The impossibility on the part of those who maintain the purely bactericidal theory of the body fluids, to reply to the objections above mentioned has accentuated this favourable movement. Just at this moment, when the theory of phagocytes might be regarded to have obtained the rights of citizenship, a discovery was made which appeared to overturn it completely. I have mentioned more than once that the attempts of the partisans of the bactericidal theory of the body fluids have failed whenever it was necessary to give evidence of their action in the refractory animal. Instead of a destruction of the micro-organisms in these fluids, it was always found that they perished inside the phagocytes. These facts have even led to the manifestation of a desire to harmonise the humoral theory with the theory of phagocytosis. Denys, with certain of his collaborators, and Buchner and his pupils came to the conclusion that the alexins are merely leucocytic products. As regards the theory of phagocytosis we have this section, who attribute an important function in healing and immunity to the emigration of the leucocytes towards, and their accumulation at the menaced spot. They admit that the leucocytes really represent the healing elements of the animal body; it is not, however, they say, their phagocytic functions which confer upon them this rôle but their power of secreting alexin. These bactericidal substances act outside the phagocytes—in the plasmas of the blood and of the exudations—and phagocytosis only intervenes at a later period and secondarily.

This new modification of the bactericidal theory of the body fluids has often been termed by Buchner a connecting bridge between the humoral theory and the cellular theory of immunity.

In the midst of this movement of conciliation, Pfeiffer[900] in 1894 published a work on the immunity of the guinea-pig against experimental cholera peritonitis. He maintains that here the destruction of the vibrios takes place without any co-operation on the part of the phagocytes and exclusively by means of the body fluids. The vibrios, before their complete destruction and solution in the fluids of the body, are transformed into granules, presenting the transformation to which we have given the name of Pfeiffer’s phenomenon.

[Sidenote: [558]]

Several of Pfeiffer’s pupils have confirmed his view in connection with the cholera vibrio, and have extended it to several other micro-organisms such as the typhoid coccobacillus. The destruction of the micro-organisms in these cases is brought about, according to Pfeiffer and his collaborators, not by the alexins of Buchner, but by a separate substance. The protective anti-infective serum contains it in an inactive state only; but immediately this serum is introduced into the body of a normal animal, the bactericidal substance is acted upon by the endothelial cells and becomes “active,” capable of destroying a large number of vibrios. Pfeiffer has developed this theory more especially in an article published in 1896, entitled “Ein neues Grundgesetz der Immunität[901].” Pfeiffer’s observation and his theory built upon it gave a new lease of life to the humoral theory and for some time many observers believed that the theory of phagocytosis was now finally overturned. Fränkel[902] announced, in a public address, that science in its progressive march has “discovered the methods of defence employed by the animal organism against its most dreaded enemies, methods which have nothing in common with phagocytosis, which act quite independently of the phagocytes and manifest an action so energetic that we may calmly eliminate all other factors.” This view is based on the discovery of antitoxins and the bactericidal substance studied by Pfeiffer.

It will be readily understood that as soon as I learnt of the existence of a real extracellular destruction of micro-organisms I at once began to study it in order to find out its real importance amongst the phenomena of immunity. First of all, I examined Pfeiffer’s phenomenon in connection with the cholera vibrio[903], and I was able to show that it was produced only under special conditions. The pre-existent phagocytes must be greatly injured before the cholera vibrios can be transformed into granules. Phagolysis (so I termed this transitory damage to the phagocytes) is indispensable for the manifestation of Pfeiffer’s phenomenon in the peritoneal fluid. When it is suppressed, by preparing the phagocytes by means of injections of various fluids, we find that, instead of Pfeiffer’s phenomenon, phagocytosis is almost instantaneously produced. In positions where very few or no leucocytes are pre-existent, as in the subcutaneous tissue, Pfeiffer’s phenomenon is never observed.

[Sidenote: [559]]

Even in the case of the cholera vibrio the extracellular destruction is observed, therefore, only in special cases. Most of the other pathogenic micro-organisms do not undergo this destructive process at all under conditions in which the cholera vibrio exhibits Pfeiffer’s phenomenon in a marked degree. These facts appeared to justify me in the conclusion that the destruction of micro-organisms takes place in the animal body by means of soluble ferments, the result of phagocytic digestion. These ferments are found under the normal condition within these phagocytes and escape from them when they are destroyed or receive some transient injury. This conclusion was in flat contradiction to the theory and statements of Pfeiffer, who attributed an important function to the endothelial secretions. To settle this controversy I tried to obtain Pfeiffer’s phenomenon outside the body, that is to say independently of any co-operation from the peritoneal endothelium. It is sufficient to add a little peritoneal lymph, rich in leucocytes, to the inactive anti-infective serum, to obtain in hanging drops the transformation of the cholera vibrios into granules.

Bordet[904], in my laboratory, repeated this experiment with the object of determining its essential mechanism. He succeeded in obtaining Pfeiffer’s phenomenon _in vitro_, not only by adding peritoneal lymph from a normal guinea-pig to the specific serum, but also by adding to it a drop of fresh blood serum from the same animal. The analysis of the phenomena which take place under these conditions led Bordet to the following hypothesis. The destruction of micro-organisms in vaccinated animals takes place by the co-operation of two substances. One of these is Buchner’s alexin which is found normally in the phagocytes; it sets up bacteriolysis properly so-called when it is enclosed within the leucocytes or after it has escaped from them at the time of phagolysis. To attain this end, however, the alexin needs the co-operation of another substance. This is the protective or sensibilising substance of Bordet. It circulates in the plasmas and carries a specific character which is absent from the alexin. I need not here insist at any length on this theory, because it has already been sufficiently explained during the course of this work.

[Sidenote: [560]]

The data on the restricted part played by Pfeiffer’s phenomenon and on its mechanism, above summarised, have been attacked by Pfeiffer and by several other observers, but they have received general confirmation, so that their accuracy can no longer be in doubt. Objections were also raised to Bordet’s view of the mechanism of bacteriolysis. Thus, Abel has criticised it in the following argument[905]: “In spite of the soundness and the boldness of the majority of Bordet’s statements on the importance of the various factors, and especially of the leucocytes in immunity, it cannot be doubted that later researches will modify and correct his interpretations which we, in Germany, do not accept in their full extension. Up to the present, the victory in the various rounds has always been with Pfeiffer, whose researches, solid and exempt from bias, have made him, to use a sporting expression, the ‘favourite’ with all those who follow attentively the international contest in the arena of the problem of immunity.” Abel is certainly a highly esteemed bacteriologist, but he is not a good prophet, and he assumes a mistaken attitude in looking at the subject from a “national” point of view[906]. In Germany much interest is taken in scientific movements and, very naturally, original and new theories are there criticised and discussed. But that does not justify one in putting forward against an opinion the statement that it is not accepted in Germany. In this country, so rich in scientific work, we find partisans of the most opposite views. In any case, in the conflict between Pfeiffer on the one hand, and Bordet and myself on the other, things have not turned out as Abel predicted. The two substances which act in the destruction of the micro-organisms are now accepted by the whole world. The intimate relations between the alexins and the leucocytes are equally recognised by very many observers. The fact that the alexins are confined within phagocytes has been confirmed by several observers, and has received a very convincing proof from Gengou’s experiments on the comparative action of the serum and blood plasma against micro-organisms. The existence of phagolysis, denied at first by some observers, has been verified by others and can now no longer be doubted.

[Sidenote: [561]]

The relations between the sensibilising substance and the phagocytes are less easily grasped than are those between the alexins and the leucocytes. Nevertheless, the experiments made by Pfeiffer and Marx[907], have led these observers to recognise that the former arise from the spleen, the lymphatic glands, and the bone marrow, that is to say, organs which are pre-eminently phagocytic. This result has been confirmed by Deutsch and must be regarded as definitely settled. All the data collected in recent years have, therefore, confirmed the view that the destruction of micro-organisms in the refractory animal presents itself as a special example of their absorption by formed elements. This truth was so fully recognised in our laboratory that the analogy between bacteriolysis and the destruction of animal cells was looked upon as quite natural and evident. Bordet had for some years past observed that the blood serum of certain animals presented a marked analogy in its agglutinative property in regard to micro-organisms and in that against red blood corpuscles. In 1898, studying the fate of the spirilla of the goose in the peritoneal cavity of guinea-pigs (see Chapter VI), I observed that these micro-organisms underwent the same changes both within and outside the phagocyte; this fact appeared to me to be in perfect harmony with the whole of our knowledge concerning the absorption of formed elements and on intracellular digestion.

Bordet[908], prepared by his preceding researches on the agglutination of the red blood corpuscles, set himself to study the fate of the red corpuscles in the animal body. He easily established a close relationship between the development of the bacteriolytic property and the haemolytic power of the serum of animals prepared by repeated injections of bacteria and of blood. His results were soon (January, 1899) confirmed by Ehrlich and Morgenroth[909], who supplemented them with the important statement that Bordet’s sensibilising substance, or intermediary substance (E. and M.), has the property of attaching or fixing itself to the red blood corpuscles.

[Sidenote: [562]]

The works on haemolysis, carried out during the last three years by Ehrlich and Morgenroth on the one hand, and by Bordet on the other, have allowed us to extend our study of the mechanism of the action of the two substances on micro-organisms and on animal cells. Ehrlich has extended his ingenious theory of antitoxins to the bacteriolytic substances, which he regards as side-chains detached from the cells and capable of absorbing the toxins. In a series of remarkable investigations, most of them carried out in collaboration with Morgenroth, Ehrlich has developed his theory which attempts to offer an account of the essential mechanism which presides over the destruction of micro-organisms and over the neutralisation of their poisons. This theory is at present in full swing of development. Some of his points contradict several of the conclusions in Bordet’s works. Whilst the latter maintains that the sensibilising substance becomes fixed as a mordant, Ehrlich regards it as entering into chemical combination with the molecular group of the micro-organisms and of the animal cells. According to Bordet, the alexin of the same species of animal is always the same substance. Ehrlich energetically maintains the plurality of the alexins, to which he gives the name of complements.

This controversy has caused a most interesting exchange of views and has led to experiments which are remarkably ingenious, but it must be admitted that as yet all the points in dispute are not definitely settled. It is evident that we have here a new line of research which promises most fruitful results for science.

We have described in various chapters of this work the fundamental elements of Ehrlich’s theory. Many think that this theory is, in principle, antagonistic to the theory of phagocytosis, but we have already observed that this view cannot be accepted. It is true that Ehrlich maintains that the bacteriolytic and cytotoxic ferments which we have called _cytases_ (alexins or complements) circulate in a state of solution in the blood plasma, whilst, according to the theory of phagocytosis, they are found under normal conditions inside phagocytes. But this view has nothing to do with the basis of the theory of receptors, or of Ehrlich’s side-chain theory, according to which the antitoxin and certain other antibodies (intermediary substance) are regarded as products detached from cells having an affinity for the toxins and the microbial products.

The theory of phagocytosis seeks to establish the part played by these cells in the destruction of micro-organisms. It maintains that the vital manifestation of the phagocytes, irritability, mobility, and voracity, constitutes an essential factor in ridding the animal of micro-organisms, because the true bactericidal ferment is contained within the phagocytes, except in cases of phagolysis. The destruction of the micro-organisms follows the laws which govern the absorption of formed elements in general. This absorption, finally, is the work of two soluble digestive ferments, one of which (fixative) is readily excreted by the phagocyte into the plasmas of the blood and exudations. The theory of phagocytosis seeks to establish these principles with the greatest possible exactness, but it has not yet ventured to penetrate more deeply into the phenomena of intracellular digestion which are confounded with the action of soluble ferments in general. This problem is still far from being satisfactorily solved.

[Sidenote: [563]]

In spite of very numerous objections, of which the principal ones have already been mentioned, the theory of phagocytosis, within the limits indicated, so far from being overturned, has become more and more consolidated, thanks to the numerous observations made since its foundation. It is for this reason that the opposition has calmed down of late years and that in many works the opinions expressed have become more favourable to the rôle of phagocytosis in immunity.

Soon after the Congress of Hygiene in 1891, the Pathological Society of London devoted several meetings to a discussion of the question of immunity. Many eminent observers took part in these debates, which were, in general, favourable to this theory of phagocytosis[910].

At the International Congress of Hygiene, held at Budapest in 1894, the question of immunity was again discussed. Buchner[911] made a report in which he specially insisted on the leucocytic origin of the alexins, regarding this fact as particularly capable of reconciling the bactericidal property of the body fluids with the theory of phagocytosis. The alexins, however, secreted by the leucocytes, must, it was assumed, carry out their principal function in the plasmas of the blood and exudations. Phagocytosis would only intervene secondarily for the purpose of ingesting the micro-organisms which had been already killed or seriously injured by the alexins of the body fluids.

[Sidenote: [564]]

In his last summary of the question, presented to the International Congress of Medicine at Paris in 1900, Buchner[912] maintains his theory of leucocytic secretions. But he already takes one step more towards the theory of phagocytosis, at least as regards natural immunity. He consents to accept the fact “that phagocytic activity is in many cases of decisive importance in overcoming the infective processes, especially in those cases in which the secreted alexins were unable to bring about more than a temporary attenuation of the vital functions of the bacteria. Under these conditions the bacteria could only be modified in so far as their chemical functions were transformed into a latent state, from which they would be ready to regain their full vital activity should it happen that the phagocytes were not there to prevent them from doing so.” In any case this view is widely removed from the old theory, according to which phagocytes were regarded as capable of ingesting dead and inoffensive bacteria only.

A second adversary of the theory of phagocytosis, von Behring[913], gives a place to this theory not only in certain examples of natural immunity but even in some cases of acquired immunity, e.g. in the immunity of sheep vaccinated against anthrax, an example I have already cited in Chapter VIII (cf. _supra_, p. 242).

It would take too long to describe the change of opinion on the theories of immunity that has taken place during recent years. I will content myself with citing certain examples which shall be taken from the works of declared adversaries of the theory of phagocytosis. Thus, Flügge, who early declared against the cellular theory completely and categorically and at the same time argued strongly in favour of the humoral theory, has been gradually led to depart from his first position. We may follow the steps of his conversion in the different editions of his _Outlines of Hygiene_. In the first edition published in 1889 he expresses himself in the following manner[914]: “Recent researches indicate the probability, however, that the phagocytes in by far the greater majority of cases seize the infective agents which, already dead, are not in a condition suitable for the performance of a defensive function. On the other hand, it is proved that the blood and blood plasma of warm-blooded animals possess the property of destroying, very quickly, enormous numbers of pathogenic bacteria,” ... etc. In the fourth edition of the same work, published in 1897, we find at the corresponding place the following passage[915]: “Recent researches indicate the probability, however, that the theory of Metschnikoff ... is not in a position to offer a complete explanation of the process of immunity.” This passage is followed by a somewhat conciliatory and eclectic development of the theory.

[Sidenote: [565]]

Let us take as a second example Günther’s _Introduction to the Study of Bacteriology_, widely read both in the original and in translations. In the first edition published in 1890[916] the theory of phagocytosis is curtly dismissed as “being incapable of withstanding criticism.” In the fifth edition of the same work, however[917], published in 1898, this theory is no longer treated thus summarily. It is given a place amongst the theories of immunity and an attempt, similar to that made by Buchner, is made to reconcile it with the humoral theory.

A change in the same direction may also be observed in Charrin’s view. In the first edition of his _Pathologie générale infectieuse_, this observer[918] had already taken an eclectic view on this question of the theories of immunity. But the function which he assigns to the phagocytes is subsidiary and secondary, whilst to that of the humoral properties is assigned a position of primary importance. In the second edition of the same work, which appeared seven years later[919], the importance of phagocytosis is recognised in a much larger measure, as may be gathered from the following passages: “For my part, I have always accepted phagocytosis: at the same time I have always accepted the existence of special humoral properties. As early as 1888 I showed, _in vivo_, that the germs are modified outside the cells; but I did not know from what groups of anatomical elements these properties were derived, I exaggerated their importance and it is the decision of this origin and this importance that renders it possible to reconcile the two theories” (p. 250). “After all, the defence rests upon these two great processes or cellular activities, phagocytosis in the first line, and then humoral influences, some of them bactericidal and injurious to the living germ, others antitoxic and injurious to their secretions” (p. 253).

[Sidenote: [566]]

Whilst the theory of phagocytosis has been consolidated by the demonstration: (1) that the phagocytes, in cases of immunity, ingest and destroy the living and virulent micro-organisms without the latter needing to be previously deprived of their toxins; (2) that the phagocytes absorb toxic substances; (3) that the phagocytes contain bactericidal cytases and produce fixatives; the humoral theories, in spite of all the efforts made to defend them, could never be developed as theories that were in the slightest degree of general application. Certain observers who from the first were very sympathetic to the humoral theories have attempted to give a complete summary of these properties. Thus, Stern[920] and later Frank[921] have published reports drawn up with great care and in a very impartial spirit on the works treating of the properties of the body fluids and the part they play in immunity. This is how they sum up the question. Stern came to the conclusion that it is impossible “to demonstrate at all regularly the existence of relations between the bactericidal action of the blood and immunity in all the infective diseases. In some cases, however, these relations are so marked that, for these examples, a causal bond between the two factors is extremely probable.” Frank expresses himself in the following manner: “It follows most clearly that the immunity of an animal—immunity innate or acquired—corresponds with the bactericidal property of the blood in certain exceptional cases only. The only animal, absolutely susceptible to anthrax and whose blood is entirely without any bactericidal power, that it is at present possible to cite, is the mouse.” “The bactericidal action of the blood serum is undoubtedly a fact of great biological importance; but equally certainly it cannot be the general cause of immunity, whether innate or acquired.”

An attempt was made to give fresh life to the humoral theory, either by assuming that the bactericidal substance is nothing but the eosinophile or pseudo-eosinophile secretion of the leucocytes (Kanthack), or by supposing that for the destruction of micro-organisms in the animal body the intervention of the agglutinative substance dissolved and distributed in the body fluids is essential (Max Gruber). These two views were put forward in a tentative form and as preliminary communications only; there is no possibility of raising them to the dignity of theories, and of late years they have not been upheld.

It cannot be denied that not one of the humoral theories has been able to retain its position or to stand against the numerous facts that have been accumulated during recent years.

This extraordinary discrepancy between the bactericidal power of the body fluids and immunity is explained by the circumstance that the microbicidal substances exist in the living animal within phagocytes and only escape from them when these cells have been injured. The fact, so well demonstrated by Gengou, that the blood plasma is without any bactericidal power has given the final blow to the microbicidal theory of the body fluids and it can no longer be maintained.

[Sidenote: [567]]

The humoral theories, based on the antitoxic and protective power of the body fluids, can claim only a very restricted application. These properties are met with in acquired immunity only, and even there are not constant. Many cases of acquired immunity against micro-organisms are unaccompanied by any antitoxic power, and in several examples of this immunity the body fluids do not exhibit any protective power.

There is only one constant element in immunity, whether innate or acquired, and that is phagocytosis. The extension and importance of this factor can no longer be denied.

It is clearly proved that phagocytes are susceptible cells which react against morbific agents, whether organised or not. These cells ingest micro-organisms and absorb soluble substances. They seize microbes whilst these are still living and capable of exercising their noxious effect and bring them under the action of their cellular contents, which are capable of killing and digesting the micro-organisms or of inhibiting their pathogenic action. Phagocytes act because they possess vital properties and a faculty of exerting a fermentative action on morbific agents. The mechanism of this action is not yet definitely settled, and we can foresee that for future researches there will be a vast and fertile field to be reached by pursuing this path.

The present phase of the question of immunity constitutes one stage only in the development of biological science and one which is capable of many improvements.