Immunity in infective diseases

When an animal remains unharmed in spite of the penetration of infective agents it is said to be immune to the diseases usually set up by these agents. This idea embraces a very great number of phenomena which cannot always be sharply separated from allied phenomena. On the one hand, immunity is closely connected with the process of cure, on the other, it is related to the disease. An animal may be regarded as unharmed if the penetration of a very dangerous virus sets up merely an insignificant discomfort. Nevertheless, this discomfort is accompanied by morbid symptoms, though they may be very slight. It is useless and impossible to set up any precise limits between immunity and allied states.

Immunity presents great variability. Sometimes it is very stable and durable; in other cases it is very feeble and transient. Immunity may be individual or it may be generic. It may be the privilege of a race, of a species.

Immunity is often innate, as is the case of the immunity which is called natural. But it may also be acquired. This last category of immunity may be developed either by natural means, after an attack of an infective disease, or as a result of human intervention. The principal means of obtaining artificial acquired immunity consists in the inoculation of viruses and of vaccines.

Immunity is a phenomenon which has existed on this globe from time immemorial. Immunity must be of as ancient date as is disease. The most simple and the most primitive organisms have constantly to struggle for their existence; they give chase to living organisms in order to obtain food, and they defend themselves against other organisms in order that they may not become their prey. When the aggressor in this struggle is much smaller than its adversary the result is that the former introduces itself into the body of the latter and destroys it by means of infection. In this case it takes up its abode in its adversary in order to absorb the contents of its host and to produce within it one or more generations. The natural history of unicellular organisms, both vegetable and animal, often presents to us these examples of primitive infection.

But infection also has its counter. The attacked organism defends itself against the little aggressor. It protects itself by interposing a resistant membrane, or it uses all the means at its disposal to destroy the invader. As a very large number of organisms, in order to obtain nourishment, are obliged to submit their food to digestion by various chemical substances, they utilise these substances in the struggle against the infective agents. They digest them whenever they are able to do so.

One of the most primitive of organisms, the plasmodium of the Myxomycetes, which is composed of formless protoplasmic masses intermediate between lower animals and plants, ingests foreign bodies of various kinds. It often happens that it incorporates numerous bacteria which are growing alongside it on rotten wood or elsewhere. The plasmodium allows them to live for some time within its digestive vacuoles. But in the end it digests them by means of its soluble ferments, substances intermediate between pepsin and trypsin. Owing to this digestive power the plasmodia are not attacked by bacterial infections.

This example, taken from amongst the most simple organisms, may serve as a prototype for the phenomena of immunity in general. At the commencement of the study of this remarkable property of so many living organisms it was thought that the pathogenic micro-organisms encountered, within the refractory organism, a medium which did not allow them to live, either because of the absence of certain nutritive substances indispensable for their existence or because it contained some substance injurious to micro-organisms. Very numerous and detailed researches have demonstrated the incorrectness of these hypotheses. There are, of course, certain pathogenic micro-organisms which are very exacting as regards the medium in which they will grow. Some will develop only in the presence of particular substances, whilst others are extremely sensitive to the slightest traces of poisons. These, however, are quite the exception. The great majority of pathogenic micro-organisms belonging to the group of bacteria readily adapt themselves to all kinds of culture media, and most of them live and develop freely in the blood or other fluids of refractory organisms. This, therefore, is not the cause of the immunity in such organisms. The cause must be sought for amongst factors more closely connected with life.

Wishing to penetrate more deeply into these phenomena the hypothesis was put forward that the unharmed organism got rid of the infective micro-organisms by expelling them to the outside along with the excreta. It was maintained for a considerable time that the animal organism possessed the means of causing pathogenic bacteria to pass into the kidneys, whence they were eliminated by the urine. It had to be acknowledged, however, that this elimination never takes place in cases of immunity, and only comes into operation when the animal is ill and the integrity of the renal filter is impaired.

The infective micro-organisms, after they have entered into the unharmed organism, remain there for a longer or shorter period, and perish without being expelled. This disappearance of the micro-organisms takes place by the same mechanism that rids the plasmodium of those bacteria which it has managed to ingest during its slow peregrinations over dead leaves or rotten wood. The micro-organisms are absorbed into the refractory organisms as the result of a true act of digestion. It is very remarkable that the gastro-intestinal ingestion, so well provided with means of rendering the most varied aliments soluble, is generally incapable of digesting pathogenic or other micro-organisms. It is very rare to meet with soluble ferments of the intestinal canal which are capable of digesting microscopic organisms, especially bacteria. Consequently this organ, so rich in digestive diastases, is generally inhabited by a large number of bacteria and other micro-organisms.

Even in animals whose food contains large numbers of microorganisms, e.g. the larvae of flies, the digestive juices are powerless to destroy them. Nevertheless, there are organisms which feed exclusively, or almost exclusively, on bacteria and which are quite capable of digesting them. These are the Protozoa, such as the Amoebae and certain Infusoria, which, without any trace of a digestive tube, easily bring about this result. Amoebae can be grown on the surface of agar by taking care to sow along with them bacteria for their nourishment. It is only necessary to give them a single species of micro-organism, and this may be selected from the pathogenic forms, such as the cholera vibrio or the Bacillus coli. The Amoebae ingest a number of these bacteria in the living state. They then kill them and digest them in their digestive vacuoles which contain, along with a little acid, a ferment belonging to the trypsin group, the amoebodiastase.

The bodies of lower and higher animals, alike, are very rich in elements which closely resemble the Amoebae. Sometimes these are to be found in the epithelial cells of the digestive canal which put out protoplasmic processes for the purpose of seizing food and transferring it to their interior, where it is submitted to the action of digestive ferments. Sometimes they are the cells disposed between the body wall and that of the intestinal canal, which float freely in the fluids of the body or are more or less fixed in the interstitial tissue. The animal kingdom presents a great variety of these amoeboid elements, known under the general name of phagocytes (cells capable of devouring solid bodies). One of the most primitive arrangements of phagocytes is met with in Ascaris and its allies belonging to the group of the Nematoda. All the organisation that these round worms possess consists merely of four, or a few more, enormous cells attached to the body wall. These are phagocytes which push out processes of enormous length, capable of exploring the whole of the internal cavity of the body.

The majority of phagocytes circulate in the lymph and blood and pass into the exudations. These white corpuscles have a comparatively uniform structure in the Invertebrata and present themselves as small cells with a nucleus and a protoplasm capable of amoeboid movements. In the Vertebrata we meet with two great categories of white corpuscles, of which one group resembles those of the Invertebrata in that they also possess a single large nucleus and an amoeboid protoplasm. These are the macrophages of the blood and of the lymph, and are intimately connected with the macrophages of such organs as the spleen, lymphatic glands, and bone marrow. Another group of white corpuscles in the Vertebrata is made up of small amoeboid cells which are distinguished by having a nucleus which, although single, is divided into several lobes. These are the microphages whose chief peculiarity, the multi-lobed form of the nucleus, must be regarded as an adaptation for the purpose of passing as rapidly as possible through the walls of capillaries and small veins.

The diapedesis of the white corpuscles, their migration through the vessel wall into the cavities and tissues, is one of the principal means of defence possessed by an animal. As soon as the infective agents have penetrated into the body, a whole army of white corpuscles proceed towards the menaced spot, there entering into a struggle with the micro-organisms. Aided by the special form of their nucleus the microphages are the first to pass through the walls of the vessels. Each of the several small lobes, into which the nucleus and its protoplasm is divided, passes readily through the minute orifices between the endothelial cells of the vessels. The macrophages follow the microphages and become mixed in greater or less numbers with the exudations. But it is not micro-organisms only which set up this inflammatory reaction accompanied by the emigration and the accumulation of leucocytes. The introduction of inert bodies and of aseptic fluids brings about the same result. The phagocytes are, as a matter of fact, endowed with a special susceptibility, which enables them to perceive exceedingly small changes in the chemical or physical composition of the medium that surrounds them.

The leucocytes, having arrived at the spot where the intruders are found, seize them after the manner of the Amoebae and within their bodies subject them to intracellular digestion. This digestion takes place in the vacuoles in which usually is a weakly acid fluid which contains digestive ferments; of these a very considerable number are now recognised.

Just as the Amoebae and the Infusoria make a choice from amongst the small organisms that surround them, so the leucocytes choose bodies which are best suited to their use. The macrophages seize by preference animal cells such as the blood corpuscles, the spermatozoa, and other elements which are derived from animals. Among the infective micro-organisms the macrophages have a predilection for those that set up chronic diseases such as leprosy, tuberculosis, and actinomycosis and also for those which are of animal nature. Into this last category come the amoeboid parasites of malaria, Texas fever and the Trypanosomata. The macrophages can also ingest the bacteria of acute diseases, but, save in exceptional cases, their intervention is of little moment.

The microphages, on the other hand, appear to play their part specially in acute infections. Their intervention against animal cells is nil, or almost so. Thus they rarely seize the red corpuscles of the same or of a foreign species of animal. They also appear to be repelled by parasites of animal origin and by certain bacteria which set up chronic diseases. Whilst the macrophages seize the bacilli of leprosy with great avidity, the microphages ingest them only exceptionally.

The morphological and physiological differences between the two great categories of mobile phagocytes (leucocytes), correspond to differences in the composition of their soluble ferments. Just as the Amoebae digest their prey by means of their amoebodiastase, a soluble ferment of the group of trypsins, so the white corpuscles submit the foreign bodies ingested by them to the action of what are now known as cytases. These cytases (alexins or complements of other writers) are soluble ferments which also belong to the trypsin group. They act in a medium which is feebly acid, neutral, or feebly alkaline, and, like the amoebodiastase, they are distinguished by a great sensitiveness to heat. When the cytases are contained in fluids, a temperature of 55°–56° C. destroys them rapidly and completely. When they are found in organs reduced to the state of an emulsion, their sensitiveness diminishes and it is necessary to raise the temperature to 58°–62° C. in order to destroy their activity.

Bordet maintains that the cytases are very different in the various species of animals, but that in the same species only one cytase exists. Ehrlich and Morgenroth, on the other hand, hold that the same serum contains several, sometimes many, different cytases. This question is too difficult to be definitely solved at present. It appears to me very probable that there exist, in the same species of animal, two different cytases. One of these, the macrocytase which is found in the lymphoid organs and in the serum of the blood, acts more particularly on animal cells. Thanks to this substance an extract or maceration of the spleen, omentum or lymphatic glands dissolves the red blood corpuscles more or less readily; these extracts and macerations, however, are incapable of destroying bacteria. When the macrophages seize the nucleated blood corpuscles they digest them completely, not sparing even the nucleus, so resistant to attack, but when the same phagocytes ingest such micro-organisms as are most easily digested, such as the cholera vibrio, their action is feeble. The vibrios, without any transformation into granules, remain alive for some time and are destroyed and digested with very great difficulty. The cytase of the microphages, or microcytase, is distinguished by other properties. It destroys and digests easily many micro-organisms, but has little or no action upon the red blood corpuscles and other animal cells. The exudations which are rich in macrophages, such as those of the lymphoid organs, are not at all or only slightly bactericidal, but exhibit a solvent action on red blood corpuscles. On the other hand, the exudations, which are composed in great part of microphages, leave red blood corpuscles intact, but readily destroy micro-organisms. Similar properties distinguish the bone marrow, extracts and suspensions of which do not dissolve red corpuscles, but attack micro-organisms. Now, we know that the bone marrow is the principal seat of origin of the microphages.

Even after the addition of some of the specific fixative to the microphagic exudations no solution of the red corpuscles is produced, which demonstrates most clearly that the microcytase is really incapable of attacking these animal cells.

We are, therefore, compelled to accept the existence of two different cytases, of which one (the macrocytase) acts specially upon elements of animal origin, and the other (the microcytase) acts principally on micro-organisms. The indication of any more detailed differentiations is impossible in the present state of our knowledge.

There are certain ferments which, during the life of the cells which produce them, pass readily into the surrounding fluids. For instance, sucrase can be recovered without difficulty from the culture fluid of moulds and yeasts. The ferments of the intestinal digestion also pass with great facility into the secreted fluids. Other soluble ferments, on the other hand, remain very closely bound up with the cells which manufacture them. Thus the zymase of the yeasts can only be freed from the cells of these fungi with great difficulty, under the influence of great pressure and under conditions which profoundly alter the cell. The proteolytic ferment of the yeast is also very adherent to the cells of these organisms. The fibrin-ferment, or plasmase of the white corpuscles, is not secreted by these cells so long as they are quite intact. But it is sufficient to subject them to unfavourable conditions of existence to cause them to throw it out from their bodies. The leucocytes, when removed from the animal, undergo a deterioration which soon leads to the deposition around them of filaments of fibrin.

The cytases must also be grouped with the soluble ferments which are not thrown off by the phagocytes so long as these remain intact. Immediately these cells are injured, however, they allow a part of their cytases to escape. In the blood, withdrawn from the animal, the white corpuscles allow the plasmase to pass into the fluid, where it sets up the coagulation of the fibrin and the formation of a clot. At the same time these cells give up some of their cytases which communicate to the serum its haemolytic and bactericidal properties. This fact is of the highest importance in connection with the question of immunity. The best demonstration of this has been furnished by a comparison of the bactericidal power in the different parts of the body and in the body fluids extracted from the animal.

When micro-organisms are introduced into those situations in the refractory animal which contain pre-existent leucocytes, the leucocytes, under the influence of the shock, undergo serious lesions, accompanied by the throwing out of the cytases. Under these conditions the least resistant micro-organisms (such as the cholera vibrio) exhibit undeniable signs of deterioration: they become transformed into granules and may even die in greater or less numbers. When, however, the leucocytes are well protected and withstand the injection of the micro-organisms without being profoundly altered, the extracellular destruction of the micro-organisms does not take place. On the contrary, a very rapid phagocytosis is produced which brings about the death and intracellular digestion of these micro-organisms. Under these conditions vibrios are also transformed into granules and perish, but only within the leucocytes. The phenomena I have just mentioned are brought about in the peritoneal cavity and in the blood vessels of refractory animals, that is to say, in situations rich in leucocytes.

In the subcutaneous tissue, in the fluids of oedemas and in the anterior chamber of the eye of these same refractory animals, the phenomena are very different. As in these situations there are no pre-existing leucocytes or their number is insignificant, the micro-organisms introduced do not suffer serious injury; they continue to live up to the moment when the leucocytes, having come up as the result of the inflammatory reaction, seize them alive, kill them, and digest them within their substance. Just as it is easy, in situations populated by pre-existing leucocytes, to suppress the extracellular destruction of the micro-organisms by preserving the phagocytes against injury or phagolysis, so this same extracellular destruction is easily set up in situations where leucocytes are absent. When, after exudations rich in leucocytes have been injected into the subcutaneous tissue, we introduce micro-organisms which are not very resistant, such as the cholera vibrio, it is observed that these vibrios are destroyed outside the cells, having first been transformed into granules.

There can be no doubt as to the conclusion to be drawn from these various experiments. The microcytase is the substance which transforms the vibrios into granules. It is within the microphages, when they remain intact, that the vibrios undergo transformation. When, on the other hand, the microphages are injured and allow the microcytase to escape, the transformation of the vibrios into granules and their partial destruction take place in the plasmas outside the phagocytes.

This conclusion is supported by comparative researches on the bactericidal power of the serum and of the blood plasma outside the animal. It is true that it is impossible to prepare a fluid which shall in all respects be comparable to the plasma of the circulating blood. There is, however, always a means of obtaining outside the animal a fluid which approaches much more closely to blood plasma than does serum. Gengou succeeded in preparing in tubes coated internally with paraffin a fluid which coagulates very tardily, and which contains very little fibrin-ferment. This fluid is found to be much less bactericidal than is the blood serum of the same animal. It is, indeed, often found to be entirely without bactericidal power, whilst the corresponding serum is capable of destroying a large number of micro-organisms.

In the phenomena of the absorption of cells also a great number of facts are met with which demonstrate that the macrocytase escapes from the macrophages at the moment of their phagolysis only. For example, the extracellular solution of the red corpuscles takes place easily in the peritoneal fluid of animals prepared by a previous injection of the same corpuscles. When the leucocytes of the peritoneal cavity are abandoned to their fate, a marked phagolysis is produced and consequently a solution of the red corpuscles in the fluid itself. When, on the other hand, phagolysis is prevented, the macrophages remaining intact do not allow their macrocytase to escape and the solution of the red corpuscles takes place almost exclusively inside the phagocytes.

In certain animals the blood serum arrests the movements of their own spermatozoa at once, whilst these remain quite motile in the animal itself. This is due to the fact that the immobilising macrocytase is contained within the macrophages and does not escape from them so long as these cells remain intact. When, in such animals, their own spermatozoa are introduced into the subcutaneous tissue, they remain motile for a long time; when, on the contrary, the spermatozoa are injected into the peritoneal cavity, where the leucocytes have not been prepared, phagolysis is produced at once and the spermatozoa become motionless immediately.

As all these data agree in demonstrating that the uninjured phagocytes retain the cytases—which remain within them, and are not found in the surrounding fluids,—we can readily understand the reason for the differences between the phenomena of immunity and the bactericidal power of the body fluids. The rat’s serum is capable of destroying a large number of anthrax bacilli, although these rodents are certainly susceptible to anthrax. The reason for this is that in the serum of the rat the bacilli are destroyed by the microcytase which is set at liberty, whilst in the body of the animal it remains enclosed within the bodies of the living microphages. So long as these cells exhibit a negative chemiotaxis against the anthrax bacillus, the micro-organism remains in the plasma, where it is not interfered with. Thanks to this, multiplication of the bacilli goes on in the body of the animal, the micro-organism killing it after becoming generalised in the blood and in the organs. The susceptibility of the leucocytes is, then, the cause of the death of the rats from anthrax, the organism of these rodents being unable to take advantage of its richness in bactericidal microcytase.

Another paradoxical fact is met with in guinea-pigs immunised against Gamaleia’s vibrio (Vibrio metchnikovi). As demonstrated by von Behring and Nissen, the blood serum of these guinea-pigs is very bactericidal for the vibrio in question. A contact of less than an hour is quite sufficient to destroy large numbers of the micro-organisms. Nevertheless, when a small dose of a culture is injected subcutaneously into these hypervaccinated guinea-pigs, the vibrios remain alive for several days, up—indeed, to the moment when they are ingested and destroyed by the leucocytes which come up in large numbers to the menaced spot. This apparent contradiction is easily explained by the fact that it is in the serum only that the vibrios encounter the microcytase, which has escaped from the microphages at the time of the formation of the clot and the separation of the serum.

Alongside those cases in which the serum of susceptible animals is found to be very bactericidal, examples are not wanting where the blood and the serum of refractory animals are entirely without this power. For instance, the pigeon is refractory to Pfeiffer’s influenza bacillus, but the blood of the pigeon forms the best culture medium for this micro-organism. The dog is refractory to the anthrax bacillus, against which the blood serum of the same animal is not at all bactericidal. The cause of this absence of parallelism between immunity and the bactericidal power of the serums must be sought in the difficulty with which the cytases escape from the leucocytes, and also in the modifications which they may undergo, once they are distributed in the fluids.

In cases of natural immunity, the cytases rid the animal of the micro-organisms without the slightest observable co-operation on the part of other soluble ferments. It is impossible to settle definitely even the question whether, in animals which enjoy this innate immunity, there exists, alongside the microcytase, any ferments which come to its aid. The conditions are quite otherwise in a very large number of cases of acquired immunity. Here it is found, as a fairly general rule, that in addition to the microcytases there exist other substances whose rôle in the defensive action offered by the animal against micro-organisms is very important. These substances are fixatives which co-operate in a remarkable fashion with the bactericidal action of the cytases; but whilst these latter injure the bacterial cell directly, the fixatives do not interfere with its life. The bacteria, permeated by fixatives, may even continue to reproduce themselves and, under certain conditions, to invade the animal. The fixatives, then, are not bactericidal, but by fixing themselves upon the micro-organisms they render them much more susceptible to the bactericidal action of the microcytases. These latter are further distinguished, in several other respects, from the cytases. The fixatives must also be classed with the group of soluble ferments, but they resist much higher temperatures than those which destroy the cytases. Whilst the latter are quite destroyed at 55° C., the fixatives, to be completely altered, must be heated to beyond 60° C. and even 65° C. On the other hand, the fixatives are distinguished by a high specificity which is never observed in the cytases. The majority of the fixatives are incapable of fixing themselves upon more than a single species of bacteria or upon a single class of animal cells, and only certain of them can fix themselves upon allied species or cells, such as the red corpuscles of several species of animals. In these cases, too, there exists a sharp quantitative difference between the fixation on the different formed elements. The same microcytases are, on the other hand, able to attack all kinds of micro-organisms, and the same macrocytases attack all kinds of animal cells.

We have seen that the cytases correspond to the zymase and to the proteolytic diastase of the yeasts in the sense that all these soluble ferments adhere with tenacity to the cells which produce them and contain them. The fixatives, in this respect, approach sucrase (invertin): these various soluble ferments pass readily into the fluids which bathe the cells that produce them. The fixatives are found not only in the blood serums, prepared outside the body, but also in the blood plasma, whence they pass into the fluids of the exudations and transudations. Whilst no cytases are found in the subcutaneous tissue, or in the clear fluids of oedemas containing no, or almost no, cells, fixatives are not absent from these various situations just indicated. For this reason, when micro-organisms are introduced subcutaneously, they are not found to be altered by the cytases, but it is easily seen that they are permeated with fixatives. The same rule applies to the fixatives of the animal cells. In the example we have cited, the spermatozoa, in an animal whose serum renders these cells motionless, remain quite motile in the epididymis and below the skin. From this fact it may be concluded that these situations contain no free macrocytase. It is sufficient, however, to add to these motile spermatozoa a drop of normal serum containing macrocytase to stop their movements at once, the fixative being well distributed in the plasma of the living animal. The spermatozoa, then, were sensibilised by the fixative which was found in both the epididymis and in the subcutaneous tissue.

The cytases are soluble ferments which are essentially intracellular: the fixatives are, on the other hand, soluble ferments which are humoral. These fixatives, however, although circulating in the plasmas, are undoubtedly of cellular origin. This fact was first demonstrated by Pfeiffer and Marx, who found the specific fixative of cholera vibrios in the “haematopoietic organs,” that is to say, in the spleen, lymphatic glands, and bone marrow, at a period when there was, as yet, none in the blood. This fact has been extended to other examples of fixatives of micro-organisms, and it cannot be questioned that the phagocytes produce these soluble ferments. Under the influence of the introduction of micro-organisms into the body, a phagocytic reaction is produced which has, as a consequence, the digestion of these micro-organisms and the production of corresponding fixatives. There is every reason to believe that, in these cases, it is the microphages which, seizing and digesting the micro-organisms, produce the fixatives.

But the macrophages are also capable of producing these adjuvant ferments. Even in normal animals the macrophagic organs, such as the spleen, and especially the mesenteric glands, contain fixatives which help in the solution of the red blood corpuscles. Into this group of facts we must also place the production by the mesenteric glands, as well as by certain other lymphoid organs, and the leucocytes of exudations and the blood, of enterokynase,—the soluble ferment which aids the digestive action of trypsin. This enterokynase is also a species of fixative; it permeates the flakes of fibrin and renders them much more accessible to the influence of the trypsins.

The fact that the enterokynase of the intestinal digestion corresponds in so many respects to the fixatives which act in the absorption of formed elements in general and of micro-organisms in particular, furnishes a further proof that the destruction of micro-organisms in the animal is an act similar to true digestion.

Phagocytes, those elements which accomplish the absorption of micro-organisms and of animal cells, those holders of digestive cytases, are also the manufacturers of fixatives. Having brought about this absorption, the phagocytes set to work to elaborate large quantities of fixatives, although they are unable to increase the amount of cytases in any marked degree. The fixatives, produced in abundance, can be excreted outside the phagocytes and pass into the blood plasma, and, with it, into the fluids of exudations and transudations. But this excretion is not an indispensable act for the functioning of the fixatives. As these ferments prepare the way for the digestive action of the cytases, it is necessary only that they should be able to fix themselves on the formed elements before the latter. It is, therefore, easy to explain cases of acquired immunity in which no fixatives are found in the body fluids. Such examples are not rare, and are characterised by the absence of any protective action on the part of the blood serum. In these cases, the fixatives, whose existence is very probable, remain lodged within the phagocytes, just as are the cytases. Within these digestive cells the fixatives may quite well fulfil their preparatory rôle, this being followed immediately by the action of the cytase. The same rule may apply also to the cases of absorption in the unprepared animal, where fixatives are not found in the blood serum, but where they are able to act within phagocytes.

The excretion of fixatives into the plasmas, which constitutes the rule in cases of acquired immunity, presents an analogy with the excretion of pepsin into the blood. This soluble ferment can and does pass habitually from the stomach into the blood and thence into the urine, where it is often met with. As the pepsin, which only acts in an acid medium, cannot be utilised in the alkaline blood plasma, it is evident that its excretion is only the consequence of a too abundant over-production.

In recent years great attention has been paid to the essential mechanism of the action of fixatives on the formed elements on the one hand, and on the cytases on the other. According to Ehrlich, the fixatives are bodies intermediate between the two. In possession of two haptophore molecular groups, they are capable of entering into chemical combination with the micro-organisms or the animal cells on the one hand, and with the cytases on the other. It is for this reason that Ehrlich applies to them the name of “amboceptors” or “intermediary substances.” Based on analogous examples in organic chemistry, Ehrlich thinks that the fixatives serve to introduce the cytases into the cells upon which they have to act. Bordet does not share this view and maintains that the action of the fixatives is not a chemical action in the proper sense of the word, but is a kind of mordanting which sensibilises the formed elements to the fermentative action of the cytases. According to him, the fixatives have no affinity for the cytases and in no way serve them as intermediaries, for which reason he gives to them the name of sensibilising substances. The question is still under discussion, but we may hope that it will soon enter into its final phase.

According to Ehrlich’s theory, the fixatives contain no product coming from the micro-organisms or from the animal cells upon which they are fixed. The fixatives are, according to him, side-chains or receptors, produced in excess and expelled into the blood plasma by the cells which produce them. Ehrlich does not tell us to what category these cells belong; he maintains only that these cells must be in possession of receptors endowed with a specific affinity for certain molecular groups of micro-organisms and of animal cells. As soon as the receptors are saturated by these molecular groups, the cells which make use of the former for their nutrition produce them in superabundant quantity. The cells of animals, treated with micro-organisms and their soluble products, or with red blood corpuscles or any other kind of element of animal origin, acquire the property of elaborating more and more of the corresponding receptors, a large proportion of which are expelled into the blood plasma.

The common point between Ehrlich’s theory and the view maintained in this work consists in the admission of a cellular property which develops more and more in proportion to the treatment of the animal by formed elements of all kinds. As, in acquired immunity against micro-organisms, the fixatives are most frequently found in the body fluids, it must be concluded that, in all these cases, the cells which produce them have become adapted by a kind of education to manufacture increasing quantities of fixatives. But even in those examples of acquired immunity where fixatives are not found in the plasmas, we must accept a modification of the cells which resist the invasion of micro-organisms. These changes in the cellular properties constitute, therefore, the most general, and consequently the most important, element in acquired immunity against micro-organisms.

As already mentioned Ehrlich does not assign any position to the cells which exhibit these modifications. It must, however, be accepted that they belong to the category of phagocytes. Indeed, the phagocytes put themselves into most intimate contact with the micro-organisms and foreign animal cells, and it is in the phagocytic organs that the fixatives are found before they are met with in the blood plasma. It may then be concluded that, in acquired immunity against micro-organisms, the phagocytes become adapted to elaborate the fixatives in large quantities, of which a portion is excreted into the body fluids, as has been shown in many examples of such immunity.

The progressive adaptation of the phagocytes in intracellular digestion can be demonstrated by the fact that in an immunised animal the fixatives are found more especially in the phagocytic organs. The leucocytes which digest gelatine exhibit in an even more distinct fashion the modification of these cells in animals which have received several injections of gelatine. The leucocytes of exudations, when the fluid is removed, become much more fitted to digest the gelatine than they were at first.

A similar adaptation is also observed in intestinal digestion, which may serve as a fresh point of comparison between the intracellular digestion of the phagocytes and the extracellular digestion in the intestines. The pancreas, in order to secrete its soluble ferments, adapts itself to the nature of the food which passes into the digestive canal.

The fixatives are not the only soluble ferments which appear in large quantities in the fluids of the immunised animal. Very often there are found along with them substances which agglutinate the micro-organisms in animals which have received several injections of micro-organisms of the same or an allied species. The same fact is observed in animals treated with animal cells. Thus the fluids of animals injected with blood corpuscles become agglutinative for these corpuscles.

The analogy between the agglutinins and the fixatives is so great that for some time several observers assumed them to be one and the same substance. This can no longer be upheld, for it is clearly demonstrated that the property of the body fluids to agglutinate micro-organisms and animal cells is different from that which brings about their permeation by fixatives. The agglutinins resist the same temperatures as the fixatives; both are specific to the same degree and pass equally from the cells which produce them into the plasmas of the blood, lymph, exudations, and transudations. The agglutinins capable of clumping the formed elements into masses may, under certain conditions, render their ingestion by the phagocytes more easy. In general, however, the part played by the agglutinins in acquired immunity must be regarded as of little importance, and for that reason we abstain from basing any theory of this immunity on the agglutinative property of the body fluids. Besides fixatives and agglutinins, the fluids of an animal which has acquired immunity very probably possess other properties which must have a greater or less function in acquired immunity. Thus, we are often struck by the stimulating action of these fluids on the normal animal into which they are introduced. This stimulation is especially manifested against the phagocytic reaction.

As, in the majority of cases of acquired immunity, the blood serum contains fixatives in considerable proportion, and as these fixatives aid the action of the cytases in a remarkable fashion, we can readily understand that the introduction of such a blood serum into a normal animal, unprepared by any vaccination, may bring about a great resistance against the corresponding pathogenic micro-organisms. The fixatives, injected with the serum, fix themselves with avidity upon the micro-organisms. These organisms may become a more ready prey to the phagocytes and be destroyed very rapidly. In particular cases, where the injection of microbial cultures sets up a phagolysis, enough cytases are thrown out to affect the microbes already sensibilised by the fixative. This is followed by a refractory condition of the animal proportionate, in general, to the amount of fixative serum that is injected. This kind of acquired immunity, conferred by serums or certain other body fluids rich in fixative substances, has often received the name of passive immunity. This term is only justified in those rare cases where the introduced serum itself contains a sufficient amount of cytases to destroy all the micro-organisms. Most often it is the normal animal which has to furnish this bacteriolytic ferment. Now, as in phagolysis the quantity given off is too small, it is to the co-operation of the holders of cytases, that is to say, to the phagocytes, that the animal must have recourse. The phagocytes, being susceptible cells, their co-operation can only be counted upon in cases where they exhibit a sufficient activity. When these elements are weakened by narcotics or by any other cause, they become incapable of intervening with efficacy and the animal falls a victim to the pathogenic micro-organisms, in spite of the more than sufficient amount of fixatives that was introduced.

In natural or acquired immunity, it is the resistance of the animal against the micro-organisms which plays the principal part. The introduction of toxins ready prepared is only done under artificial conditions, as in laboratory experiments. Hence we see that, under natural conditions, it is against the penetration of the micro-organisms that the animal must be protected. So soon as these producers of poisons can no longer maintain themselves in the immunised animal their toxic secretions do not come into play. It is for this reason that animals vaccinated against pathogenic micro-organisms do not suffer from intoxication, although they are by no means insusceptible to the microbial poisons. It is a fact of the highest importance from the point of view of immunity in general, that the resistance offered to micro-organisms in no way implies insusceptibility to their poisons. The view has frequently been expressed that, in acquired immunity at least, the animal must first acquire immunity against the microbial toxins, after which the micro-organisms, deprived of their principal weapon, descend to the rank of inoffensive saprophytes. Such cases may be found, but it is none the less true that immunity against micro-organisms may be acquired independently of that against the toxins, and that this constitutes the general rule.

Immunity is much more readily acquired against micro-organisms than against their toxins. Hence, antimicrobial vaccination was accomplished by science before that against their toxins. In the early researches on this subject antitoxic immunity appeared to be very difficult of attainment, and it was only after the discovery made by von Behring, who inaugurated a new path in microbiology, that better results were obtained. Von Behring not only succeeded in immunising animals against some of the principal microbial toxins, he demonstrated the existence of specific antitoxins in their body fluids.

This very unexpected conception of antitoxins at once took root in science, for it has been possible, thanks especially to the remarkable works of Ehrlich, to extend it to toxins of non-microbial origin. We are already acquainted with a certain number of antitoxins which, however, are not comparable in number to the other antibodies. Amongst these, the fixatives have many points of analogy with the antitoxins. Like them, they are resistant to heat: they exhibit also a fairly marked specificity, and, like the fixatives, they are distributed in the plasmas.

In the presence of so many points of similarity with the fixatives, one is tempted to attribute to the two categories of antibodies the same origin. The elaboration of antitoxins by the phagocytic elements, accumulated in the blood and disseminated in the organs, appears, in fact, to be very probable. Certain facts bearing on the absorption of various toxins by the leucocytes, as well as the distribution of antitoxins in the animal body, speak in favour of this view. On the other hand, the impossibility of attributing the elaboration of antitoxins to cells attacked by the corresponding toxins is quite in harmony with the same hypothesis. This hypothesis is especially supported by the numerous facts which prove the readiness with which the leucocytes react against all kinds of poisons, microbial or other toxins, as well as against organic and mineral poisons, such as the alkaloids and the arsenical combinations. However, in spite of so many data which speak in favour of the phagocytic origin of antitoxins, it has been impossible to support this view by rigorous facts easy of interpretation, such as those which science possesses in support of the phagocytic origin of fixatives.

The antitoxins have acquired a very great importance in the artificial cure of toxo-infective diseases, the aim in these cases being to paralyse the action of the toxins already produced by the micro-organisms and absorbed by the diseased animal. But their function is less in the protection against diseases where the object to be obtained is a reaction against the micro-organisms before these are able to inundate the animal with their toxic secretions. It is for this reason that the immunity against toxins must, in the study of immunity, occupy a less preponderant place than does the immunity against micro-organisms.

As the micro-organisms placed in the refractory animal ultimately undergo a digestion by chemical substances elaborated by the phagocytes, so also the toxins undergo a chemical modification due to the presence of substances in the production of which the living elements of the animal play a large part. The direct action of antitoxins on the toxins, so well demonstrated, especially by Ehrlich’s investigations, does not, however, exclude the intervention of living cells, which, though sometimes not very manifest, is in other cases very marked.