Paul Ehrlich’s Side-Chain Antibody Theory (1900) Part 1

In 1900, Paul Ehrlich, considered the “Father of Chemotherapy,” took a shot at creating a theoretical explanation for how immunity occurs inside the human body. According to Ehrlich’s Nobel Prize biography, while working with Behring’s anti-diphtheria serum, Ehrlich was said to have standardized this serum in units related to a fixed and invariable standard. Previously, his work had shown that the anti-toxin in the sera varied so much due to numerous factors that it could not be measured. This unit of standardization became the basis for all future standardization of sera. Ehrlich’s work on these substances eventually led him to create the Side-Chain theory of immunity which introduced the idea of unseen antibodies, antigens, and the concept of the lock and key mechanism of action:

The Contributions of Paul Ehrlich to Pharmacology: A Tribute on the Occasion of the Centenary of His Nobel Prize

“With the aim of proposing a plausible explanation for the process of immunity, it was in 1897 when Ehrlich formulated his ‘side-chain theory’, which became the basis for his immunological research at the time. This theory postulated that cells present on their surface a set of side-chains to which Ehrlich attributed functions related with the assimilation of metabolic products. A side-chain from a given cell might have, by simple coincidence, a molecular structure that allowed it to bind with a specific toxin corresponding to diphtheria, tetanus, or some other microorganism. This strictly specific binding between the toxin and the side-chain, in a manner similar to the ‘lock-and-key’ model for enzymes and their substrates described in 1894 by Hermann Emil Fischer (1852–1919), would mean the cell lost its normal function, a phenomenon which in turn would trigger the production of additional side-chains. A large part of these newly produced excess side-chains would be released into the blood stream, where they would act as antibodies or antitoxins upon binding to the toxin present in the blood, and would thus prevent the toxin from binding to other cells in the organism. Thus a small amount of toxin could produce a large amount of antitoxin able to neutralize the toxin’s own effect [13].

In 1900, Ehrlich introduced the term ‘receptor’ as a substitute for the term ‘receptive side-chain’. This idea led to a more functional concept that could be applied to the field of pharmacology. John Newport Langley (1852–1925) (table (table1),1), who studied the effect of alkaloids on muscle cells, proposed the existence of receptors that could be blocked by antagonists or activated through the action of agonists. Although they came from very different research backgrounds, the interaction between Ehrlich’s and Langley’s thinking was highly productive. Together they developed the receptor theory and extended the concept of the receptor to ‘chemoreceptors’ to describe the interaction between drugs and cells. Thus the side-chain theory, although not entirely accurate, gave rise to concepts which were to become basic tenets in immunology [6, 13, 17].

In addition to the side-chain and receptor theory, Ehrlich also established the concepts of active and passive immunity, as well as the mechanisms of transmission of immunity from mother to fetus. However, he was not without detractors who criticized him for his excessively lively imagination and lack of self-criticism. He himself compared his side-chain theory to the cellular theory of Rudolf Virchow (1821–1902), which had also met resistance initially.”

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2790789/

Ehrlich aimed to create a “plausible explanation” for the phenomena that he and other researchers were unable to observe. He worked with them to dream up and formulate the concepts and even though his ideas were not “entirely accurate,” these inaccurate fantasies became the foundation for Immunology. Ehrlich was criticized by his peers for his lively imagination and upon reading his theory, you will see why. Depending on where you look, there are differences of opinion on just how accurate Paul Ehrlich’s Side-Chain theory of immunity truly was.

It was either entirely correct and laid the foundation for what we know about antibodies today:

“Ultimately all aspects of Ehrlich’s theory would be proven correct with the minor exception that the “receptor” exists as both a soluble antibody molecule and as a cell-bound receptor; it is the soluble form that is secreted rather than the bound form released.”

https://en.m.wikipedia.org/wiki/Side-chain_theory

Or was somewhat correct:

“Two of Ehrlich’s postulates, that antibodies are identical to the antigen receptors and that antigen binding triggers the synthesis of antibody with the same specificity as the receptor, are now known to be essentially correct.”

https://www.ufrgs.br/imunovet/molecular_immunology/historyi.html

Or was wrong and completely abandoned:

“But this theory was abandoned when Landsteiner demonstrated that antibodies are also produced against various synthetic chemicals besides natural antigens.”

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=61812

Whatever the case may be, it is clear Ehrlich’s 27-page presentation brought about many of the concepts used today when discussing immunity. However, it is also clear that Ehrlich created a fictional account of what occurs inside the body based on indirect evidence from multiple researchers over multiple years to explain an unobservable phenomena. As it is a long paper covering many aapects of his theory, I have broken it down into two parts. This first section deals with the first 18 pages which contain the bulk of Ehrlich’s Side-Chain theory. The second part will be in a separate post and will look more specifically at the complement system. The first 18 pages are presented below with a summary at the end:

On Immunity with Special Reference to Cell Life.

“Honoured President, my lords and gentlemen,—It is to me the very greatest honour that I have been summoned here by your most highly esteemed Society, which for more than two centuries has represented and still represents the centre of the scientific life of England, in order that I may deliver the Croonian Lecture. I consider I am not so much personally concerned in the honour that you bestow on me, and that I shall not err if I see in it a recognition of the scientific path which I, in company with many others, have sought to follow, and which in your eyes suffices to place the field in which I work on a footing alongside of exact science. It is an extreme pleasure for me to have the privilege of addressing so many medical colleagues with whom for so many years I have been bound in close ties of friendship, and who have always been the first to welcome and to give recognition to the results of my work.

Since Jenner made his great discovery of the protective action of vaccinia against small-pox, a century has passed away. During these years that terrible scourge of mankind has been almost completely eradicated from the civilised world. The beneficial consequences of Jenner’s discovery are so evident to all who have any wish to properly appreciate them, that one wonders why, during so great a portion of the long period of 100 years, they were allowed to stand alone, without any endeavour being made to induce an artificial immunity in the case of other infectious diseases. This is all the more remarkable because Jenner’s discovery demonstrated in their entirety those essential principles which, in later times, have been established for other infectious diseases.

In the first place, it was shown that by the use of an attenuated virus, which of itself was non-injurious to the organism, it was possible to ward off the disease caused by the virulent virus. Jenner also established—what is most important from the practical
point of view—that by the inoculation of the weakened poison there was produced not only an immediate, but also an enduring, protection. That Jenner’s discovery remained so isolated was due essentially to the fact that the theoretical conceptions of the cause and nature of infectious diseases made no advance during the subsequent decades; indeed, it would be an interesting topic for some
historian of medicine to trace step by step the gradual advance in the knowledge of infectious diseases during the past century. Schwann’s
classical investigations must be regarded as the first link in the long chain. Schwann it was who, in an unsually brilliant manner, first demonstrated that the decomposition of organic bodies in the processes of fermentation and putrefaction was never spontaneous, but constantly arose through the agency of micro-organisms coming from without. This line of investigation reached its zenith in the fundamental work of Pasteur, of which the first and the greatest result—Lister’s method of wound treatment—worked a revolution in surgery. Then followed the profound investigations of Koch on Anthrax, and the pure cultivation of the most important pathogenic bacteria.

The work of Pasteur and of Koch afforded the first basis on which the study of artificial immunity could be again undertaken. The possibility of voluntarily producing a number of the most important infectious diseases of men and animals, and of modifying at will pure cultivations of bacteria, either, according to Jenner’s precedent, by passage through the animal body, or otherwise in artificial culture media, laid the foundation on which advancement could proceed. Pasteur himself was the first, after Jenner, to produce an artificial immunity by using an attenuated virus; and he was also able to introduce the procedure to some extent into practice with most beneficial results. Still the theoretical explanations of all these facts lagged far behind their practical effects. The very able investigations of Metchnikoff and his theory of phagocytosis were, to many investigators, inconclusive.

With Behring’s discovery, that in the blood serum of animals immunised against diphtheria and tetanus, there were contained bodies which were able to specifically protect other animals against the toxines of these diseases, an altogether new factor was introduced into the question. This remarkable discovery seemed at one stroke to open up an entirely new and” extremely promising prospect of immunising mankind against the majority of the infectious diseases. It was, therefore, somewhat disappointing when there did not follow, on the successful practical application of diphtheria antitoxic serum, a rapid succession of similar achievements. It may with truth be said, that during recent years there has been somewhat of a standstill in the further following-out of a work at first so enthusiastically received. By purely empirical methods, e.g., by the production and use of sera of very great antitoxic value, the results attained showed no improvement. Better success was only to be hoped for when by an accurate knowledge of the theoretical considerations underlying the question of immunity, explanations of the previous ill-success were forthcoming. Impelled by these considerations I laboured for years trying to shed some light into the darkness that shrouded the subject.

In all exact work with chemical bodies—for only as such can we regard the toxines produced by the living bacteria—the first desideratum in the investigation is the exact numerical determination of action and counteraction. The words of the gifted natural philosopher Clerk Maxwell, who said that if he were required to symbolise the learning of our time he would choose a metre measure, a clock, and a kilogramme weight, are equally apposite in reference to progress in the field of inquiry in which we are at present interested. And so at the very beginning of my theoretical work on immunity I made it my first task to introduce measures and figures into investigations regarding the relations existing between toxine and antitoxine. From the outset it was clear that the difficulties to be overcome wrere extremely great. The toxines, i.e., the poisonous products of bacteria, are unknown in a pure condition. So great is their potency, that we are obliged to assume that the strongest solid (feste) poisons which are obtained by precipitating toxic bouillon with ammonium sulphate, represent nothing more than indifferent materials, peptones and the like, to which the specific toxine attaches itself in mere traces beyond the reach of weighing; for up to the present time, by the purely chemical methods of weighing and measuring, it has been impossible to ascertain anything as to their presence or the intensityof their action.

Their presence is only betrayed by the proof of their specific toxicity on the organism. For the exact determination, e.g., of the amount of toxine contained in a culture fluid, the essential condition was that the research animals used should exhibit the requisite uniformity in their susceptibility to the poison. Uniformity is not to be observed in the reaction of the animal body to all toxines. Fortunately in the case of one important body of this nature, viz., the diphtheria toxine, the conditions are such that the guinea-pig affords for investigations the degree of accuracy necessary in purely chemical work. For other toxines this accuracy in measuring the toxicity cannot be attained. It was necessary for me to try to eliminate, as far as possible, the varying factor of the animal body, and bring the investigations more nearly into line with the conditions necessary for experiments of a chemical nature. In the course of these endeavours it was shown that it was possible to obtain in a comparatively simple manner an insight into the theoretical considerations necessary to a proper understanding of immunity, by means of test-tube experiments with suspended animal tissues. The relations were simplest in the case of red blood corpuscles. On them, outside the body, the action of many blood poisons, and of their antitoxines, can be most accurately studied, e.g., the actions of ricin, eel-serum, snake-poison, tetanus toxine, &c. In an experiment of this kind, in which are employed a series of test-tubes containing definite quantities of suspended blood corpuscles, each test-tube represents as it were a research animal, uniform in any one series, and one that can be reproduced at will. By means of these test-tube experiments, particularly in the case of ricin, I was able, in the first place, to determine that they yielded an exact quantitative representation of the course of the processes in the living body. The demonstration of this fact formed the basis of a more extended application of experiments of this nature. It was shown that the action of toxine and antitoxine took place quantitatively as in the animal body. Further, these experiments yielded a striking series of facts of importance for the theoretical valuation of the reaction between toxine and antitoxine. It was proved in the case of certain toxines—notably tetanus toxine—that the action of antitoxines is accentuated or diminished under the influence of the same factors which bring about similar modifications in chemical processes—warmth accelerates, cold retards the reaction, and this proceeds more rapidly in concentrated than in dilute solutions. These facts, first ascertained by means of test-tube experiments, have since been confirmed by Behring and Knorr for tetanus within the animal body, and by Martin and Cherry in the case of snake-venom.* The knowledge thus gained led easily to the inference that to render toxine innocuous by means of antitoxine was a purely chemical process, in which biological processes had no share. Yet again insurmountable obstacles seemed to present themselves to this conclusion.

It must be postulated that in chemical processes the bodies sharing m the action react with one another in definite equivalent quantities. This proposition appeared, however, not to hold in the case of the action of antitoxine on diphtheria toxine. When, in the case of diphtheria toxines of different stocks, that quantity of toxine bouillon which is exactly neutralised by a certain definite quantity of diphtheria
antitoxine (the official German immunity unit, as laid down for the control examination of sera), was determined, so that every trace of toxic action was abolished, the figures obtained were not in accord. Of one toxine bouillon 0’2 c.c., of another 2-5 c.c., were so neutralised by one immunity unit. Such a relation need not have given rise to surprise, because it was well known that the diphtheria bacillus, according to outside circumstances, yields in the bouillon very different quantities of toxine. It was therefore allowable to infer that the different quantities of toxine bouillon, which were saturated by one immunity unit, were exact expressions of the toxicities of the
various bouillons, or, to use other words, indifferently whether the bouillon was strongly or feebly toxic, the same multiple of the minimal lethal dose would be constantly neutralised by one immunity unit, so that in every case the law of equivalent proportions would hold good.

But when looked into more closely, the relations showed themselves to be by no means so simple. In what manner could one obtain a satisfactory estimation of the strength of a toxine? As the constant factor in such an estimation, it was only possible to proceed from a previously determined standard reaction in the case of a definite species of animal, and so we came to regard as the “toxic unit” that
quantity of toxic bouillon which exactly sufficed to kill, in the course of four days, a guinea-pig of 250 grammes weight.

When we employed this standard unit, or “simple lethal dose,” to estimate the amount of toxic bouillon neutralised by one. “immunity unit,” the facts which presented themselves were far more surprising than it was possible to have foreseen at the outset. These results were, that of one toxine, perhaps 20, of a second, perhaps 50, and of yet a third, it might be 130 simple lethal doses were saturated by one immunity unit. Since, however, we had previously assumed that the simple lethal dose alone afforded a standard on which reliance could be placed in determining the combining relations of toxine and antitoxine, it appeared from these results that the neutralisation of toxines by antitoxines did not follow the law of equivalent proportions, and, notwithstanding all earlier work in agreement with such a conception of the action, we were obliged to conclude that between toxine and antitoxine a purely chemical affinity did not exist. The seemingly inexplicable contradiction between the results just stated and previous work was very soon explained. When the neutralisation point of toxine and antitoxine was investigated for one and the same sample of poison, the following results were obtained. Immediately on its preparation, fresh from the incubator, it was found that one immunity unit neutralised a c.c. of toxic bouillon, and this quantity represented (3 simple lethal doses. When the same toxic bouillon was examined after a considerable interval, the remarkable fact was discovered that exactly a c.c. of the toxic bouillon were again neutralised by one immunity unit; but that these a c.c. now represented only simple lethal doses. It therefore followed that the toxic bouillon had retained exactly the same combining affinity, but possessed feebler toxicity. From this it was evident that the toxic action on animals and the combining capacity with antitoxine represented two different functions of the toxine, and that the former of these had become weakened, while the latter had remained constant.

Treated from the chemical standpoint, this circumstance was most simply explained by assuming that the toxine was characterised by the possession of two different combining groups: one, which may be designated haptophore, conditions the union with antitoxine, while the other group, which may be designated toxopliore, is the cause of the toxic action. From the constancy of the combining capacity, and the diminution in the toxicity, it was to be inferred that the toxophore group was very unstable, but the haptophore group more stable, and also that the deterioration of the toxophore group proceeded of necessity quite independently of any relation to the haptophore group.

If we now designated a toxine molecule, of which the toxophore group is destroyed, but its haptophore group retained, as “toxoid,” then the above-described process will represent the quantitative progress of the conversion of the toxine molecules into toxoid molecules. Such a toxoid molecule has the same quantitative combining affinity for antitoxine as the original toxine molecule, in spite of the disappearance of toxicity to the animal body. In other words, the affinity of the haptophore group for the antitoxine is absolutely independent of the existence of a toxophore group. Also, in the original toxine molecule, both groups must be to such a degree non-related or independent of one another, that a mutual reaction between them does not take place. This conception of the constitution of diphtheria toxine, after more extensive, very exact, and much varied experimentation, based on its partial neutralisation by antitoxine, has been confirmed in the completest manner possible. At this time it would be superfluous for me to enter into all the details pertaining to these investigations. It need only be remarked that in principle the same relations have been established for tetanolysine by Madsen, for snake-poison by Meyers, and for the milk-curdling ferment by Morgenroth.

The separation of the characteristic atom groups of the toxine molecule into a haptophore and a toxophore group, afforded not merely a satisfactory chemical explanation of the process of neutralisation: the possession of the knowledge of the existence of these groups yielded us, at the same time, the key to the nature of the toxic property of toxines, and to the mystery of the origin of the antitoxines themselves. After it had been established by the already described method, that the toxine molecule was possessed of a definite haptophore group, which accounted for its capacity to enter into combination with other bodies, it was immediately necessary to inquire into the question whether, and if so to what degree, this group entered into the causation of the symptoms of illness. That chemical substances are only able to exercise an action on the tissue elements with which they are able to establish an intimate chemical relationship is a conception of a general nature, which has been entertained since the birth of scientific medicine.

It is astonishing, almost astounding, that this axiom, of which the theoretical importance has been so long recognised, and which has served indeed as the first ground for certain therapeutical procedures, 
should as a matter of fact have played in the building up and furtherance of scientific pharmacology a role so insignificant in proportion to its great importance. In glancing through the modern text-hooks of pharmacology, with rare exceptions, as, e.Stokvis, one finds absolutely no mention of the distribution of drugs in the organism, a matter which is of so much moment for arriving at a true comprehension of the relations existing between pharmacological action, location in the organism, and chemical constitution. As a matter of fact, the methods for obtaining any knowledge of the exact distribution of drugs in the body are as yet very imperfect. Even if we can prove that certain alkaloids are again recognisable as being, e.g., present in the brain, we are but little further advanced in our knowledge of the process, because we cannot determine in which cells and which system of fibres the alkaloid is localised.

I may say, indeed, that as yet the investigation of the laws pertaining to the minute distribution of a chemical substance in the body is only possible when, as in the case of coloured bodies, these are at once recognisable by the eye. But that it is possible at once to draw conclusions of therapeutic importance from the laws governing the distribution was shown in the case of methylene-blue, in which I was able, knowing its distribution in the body, to anticipate for it certain antineuralgic and antimalarial properties which were both established by subsequent investigation. It may be permitted me to call to mind, that in malaria methylene-blue is especially of service in the case of persons who, on account of susceptibility, cannot be treated with quinine, and that in the hands of Koch it has shown itself of eminent value in hsemoglobinuric fever, since as opposed to quinine it exercises no destructive action on the erythrocytes. If we are not able to discover the principles governing the localisation of common chemical bodies, which can be used in suitable quantities in chemical purity, and which chemical and other reactions render perceptible, it was a priori very unlikely that efforts directed to locating the toxines, which are potent in the slightest traces, and which are bodies we have no means of rendering perceptible to our senses, would be anything else than absolutely without result.

But that this is not so, has been shown by experiments carried out by Professor Donitz, in the Steglitz Institute, to which, on account of their great importance, I shall refer somewhat extensively. When a rabbit receives a suitable dose of diphtheria or tetanus toxine injected directly into the circulation, the animal remains for many hours well, and then begins to show symptoms of illness, which gradually increase till they end in death. In order to arrive at an explanation of the incubation period, Donitz determined the amount of antitoxine which, injected intravenously immediately after the toxine, absolutely neutralised the latter. This neutralising dose is able to render all the toxine circulating in the blood innocuous. When, however, the neutralising dose so determined was injected not immediately, but seven or eight minutes after the injection of the toxine, death occurred from tetanus exactly as if no antitoxine had been given. Part of the toxine, equal at least to the minimal lethal dose, must within this time have disappeared from the blood, in which it would have been neutralised, and passed over to the tissues, especially to the brain. The experiments of Donitz were afterwards confirmed by an investigation conducted in quite a different manner by Heymans, who showed that a research animal from which the blood had been removed immediately after the injection of the minimal lethal dose of tetanus toxine, and replaced by transfusion of fresh blood, succumbed from typical tetanus. In this case, therefore, in that brief interval of time the minimal lethal dose of toxine had passed through the walls of the vessels and been taken up by the tissues.

Regarding the nature of the processes here concerned, a satisfactory explanation was also afforded by the experiments of Donitz. It admitted of demonstration that the toxine held in the tissues could still be withdrawn from them, if not the simple neutralising dose were injected but larger quantities of the same.

The quantity necessary was greater in proportion as the interval elapsing after the injection of the toxine was longer. However, after a definite period was exceeded, all possible doses of antitoxine, even the very greatest, were impotent, notwithstanding that the animal at the time of the injection of the antitoxine had not developed any symptoms. Since a very great number of other chemical substances, narcotics, alkaloids, and other neurotropic bodies, were not in a position to withdraw the toxine once deposited in the central nervous system, and as the property to do so was solely the characteristic of the specific antitoxine, one was obliged to come to the conclusion that the union between the toxine and the tissues, which could only be overcome by means of a specific chemically-related antagonising agent, must itself depend on a chemical combination. One was therefore forced to accept the idea that the central nervous system, that is to say certain ganglion cells in it, possessed atom groups resembling those of the antitoxine, in having a maximum affinity for tetanus poison. The predilection of the nervous system for tetanus toxine, the rapid union of the toxine with the nervous tissue, the gradual onset of the symptoms and their long duration could only be explained by the existence of such toxophil groups. The statement of Donitz that the tetanophile atom groups are in the guinea-pig essentially confined to the central nervous system, whereas in the case of other species, especially rabbits, these are also present in other organs, is one of prominent importance.

The beautiful experiments of Roux on intracerebral injection of toxine have yielded absolute confirmation of the statement of Donitz. Roux found in guinea-pigs that the same dose of tetanus toxine was lethal, whether given by intracerebral or by subcutaneous injection for rabbits, however, the lethal dose was twenty times greater subcutaneously than it was in intracerebral injection. This can only be explained in the terms of Donitz’s observation, viz., that in the case of direct injection of the toxine into the brain, the toxophile atom groups there present at once seize on all the toxine, while when the toxine is administered through the blood stream, the toxophile groups present in other organs also take up the toxine in equivalent quantities. In the case of rabbits the absorption of the toxine in this way is very considerable: indeed of twenty parts only one part finds its
way into union with the nervous system.

We now come to the important question of the significance of the toxophile groups in organs. That these are in function specially designed to seize on toxines cannot be for one moment entertained. It would not be reasonable to suppose that there were present in the organism many hundreds of atomic groups destined to unite with toxines, when the latter appeared, but in function really playing no part in the processes of normal life, and only arbitrarily brought into relation with them by the will of the investigator. It would indeed be highly superfluous, for example, for all our native animals to possess in their tissues atomic groups deliberately adapted to unite with abrin, ricin, and crotin, substances coming from the far distant tropics.

One may therefore rightly assume that these toxophile protoplasmic groups in reality serve normal functions in the animal organism, and that they only incidentally and by pure chance possess the capacity to anchor themselves to this or that toxine.

The first thought suggested by this assumption was that the atom groups referred to must be concerned in tissue change; and it may be well here to sketch roughly the laws of cell metabolism. Here we must in the first place draw a clear line of distinction between those substances which are able to enter into the composition of the protoplasm, and so are really assimilated, and those which have no such capacity. To the first class belong a portion of the food-stuffs par excellence; to the second almost all our pharmacological agents, alkaloids, antipyretics, antiseptics, &c.

How is it possible to determine whether any given substance will be assimilated in the body or not. There can be no doubt that assimilation is in a special sense a synthetic process—that is to say, the molecule of the food-stuff concerned enters into combination with the protoplasm by a process of condensation involving loss of a portion of its water. To take the example of sugar, in the union with protoplasm, not sugar itself as such but a portion of it comes into play, the sugar losing in the union some part of its characteristic combining reactions. The sugar behaves here as it does, e.g., in the glucosides, from which it can only be obtained through the agency of actual chemical cleavage. The glucoside itself yields no trace of sugar when extracted in indifferent solvents. In a quite analogous manner the sugar entering into the constitution of albuminous bodies (glycoproteids) cannot be obtained by any method of extraction; at least, not until chemical decomposition has previously taken place. It is therefore generally easy, by means of extraction experiments, to decide whether any given combination in which cells take part is or is not a synthetic one. If alkaloids, aromatic amines, antipyretics, or aniline dyes be introduced into the animal body it is a very easy matter, by means of water, alcohol, or acetone, according to the nature of the body, to remove all these substances quickly and easily from the tissues. This is most simply and convincingly demonstrated in the case of the aniline dyes. The nervous system stained with methylene blue, or the granules of cells stained with neutral red, at once yield up the dye in the presence of alcohol. We are therefore obliged to conclude that none of the foreign bodies just mentioned enter synthetically into the cell complex; but are merely contained in the cells in their free state. The combinations into which they enter with the cells, and notably with the not really living parts of them (Kupffer’s paraplastic portions), are very unstable, and correspond usually only to the conditions obtaining in solid solutions, while in other cases only a feeble salt-like formation takes place. I myself in 1887 placed on a sure footing the fact that the nervous system and the fatty tissues allow of alkaloids and aniline dyes being mechanically shaken out of them, as in the poison-detection process of Stas and Otto.

Hence with regard to the pharmacologically active bodies in general, it was not allowable to assume that they possessed definite atom groups, which entered into combination with corresponding groups of the protoplasm. This corresponds, as I may remark beforehand, with the incapacity of all these substances to produce antitoxines in the animal body. We must therefore conclude, that only certain substances, food-stuffs par excellence, are endowed with properties admitting of their being, in the previously defined sense, chemically bound by the cells of the organism. We may regard the cell quite apart from its familiar morphological aspects, and contemplate its constitution from the purely chemical standpoint. We are obliged to adopt the view, that the protoplasm is equipped with certain atomic groups, whose function especially consists in fixing to themselves certain food-stuffs, of importance to the cell-life. Adopting the nomenclature of organic chemistry, these groups may be designated side-chains. We may assume that the protoplasm consists of a special executive centre (Leistungs-centrum) in connection with which are nutritive side-chains, which possess a certain degree of independence, and which may differ from one another according to the requirements of the different cells. And as these side-chains have the office of attaching to themselves certain food-stuffs, we must also assume an atom-grouping in these food-stuffs themselves, every group uniting with a corresponding combining group of a side-chain. The relationship of the corresponding groups, i.e., those of the food-stuff, and those of the cell, must be specific. They must be adapted to one another, as, male and female screw (Pasteur), or as lock and key (E. Fischer). From this point of view, we must contemplate the relation of the toxine to the cell.

We have already shown that the toxines possess for the antitoxines an attaching haptophore group, which accords entirely in its nature with the conditions we have ascribed to the relation existing between the food-stuffs and the cell side-chains. And the relation between toxine and cell ceases to be shrouded in mystery if we adopt the view that the haptophore groups of the toxines are molecular groups, fitted to unite not only with the antitoxines but also with the side-chains of the cells, and that it is by their agency that the toxine
becomes anchored to the cell.

We do not, however, require to suppose that the side-chains, which fit with the haptophore groups of the toxines, the side-chains which are toxophile, represent something having no function in the normal cell economy. On the contrary, there is sufficient evidence that the toxophile side-chains are the same as those which have to do with the taking up of the food-stuffs by the protoplasm. The toxines are, in opposition to other poisons, of highly complex structure, standing in their origin and chemical constitution in very close relationship to the proteids and their nearest derivatives. It is, therefore, not surprising if they possess a haptophore group corresponding to that of a food-stuff. Alongside of the binding haptophore group, which conditions their union to the protoplasm, the toxines are possessed of a second group, which, in regard to the cell, is not only useless but actually injurious. And we remember that in the case of the diphtheria toxine there was reason to believe that there existed alongside of the haptophore group another and absolutely independent toxophore group.

Now for certain cellular elements of the body it can be proved in the test-tube that between these tissues and certain toxines an “anchoring” process takes place exactly similar to that between toxine and antitoxine. Wassermann first demonstrated this in the case of the brain substance. In a mixture of tetanus toxine and broken-down fresh guinea-pig brain the latter so bound or “anchored” the toxine that not only was the surrounding fluid toxine-free, but the brain substance laden with the tetanus toxine had also lost its own toxic action, and so the mixture when injected into an animal was borne without any harm.

The deduction is, that in this case, a chemical union between the brain substance and the tetanus toxine had taken place, and this was of so firm a nature that on introduction into the body the union was not broken up and therefore the toxine remained innocuous. The brain of the normal animal had, in keeping with my theory, acted exactly like a real antitoxine. There are present in the brain, i.e., in the ganglion cells, tetanophile protoplasmic groups, which unite themselves with the toxine. The presence of such groups is the necessary preliminary and cause of the poisonous action of the tetanus toxine in the living animal. That the process here was not one of simple absorption is proved by the fact that, if the group concerned was destroyed by heat, the brain substance became as incapable of removing the toxine as an emulsion of any other organ of the guinea-pig.

As has been said, the possession of a toxophile group by the cell is the necessary preliminary and cause of the poisonous action of the toxine. This can be most sharply demonstrated in the case of certain blood poisons, viz., the hsemolysines, which exercise a solvent action only on such red blood corpuscles as are able to unite chemically with them. The union with the red corpuscles can be proved, and one has here the great advantage of dealing with living and intact red blood cells instead of broken-down cellular material. Under these conditions it is easy to determine the quantitative relations of the union. If we now regard the action of the toxines with which we are concerned in accordance with the views we have just been discussing, we are obliged to conclude that these are only in a position to act prejudicially on the organism if they are able, by means of their haptophore groups, to anchor themselves to the side-chains of the cells of organs essential to life. If the cells of these organs lack side-chains fitted to unite with them, the toxophore group cannot become fixed to the cell, which therefore suffers no injury, i.e., the organism is naturally immune. One of the most important forms of natural immunity is based upon the circumstance, that in certain animals the organs essential to life are lacking in those haptophore groups which seize upon definite toxines. If, for example, the ptomaine occurring in sausages, which for man, monkeys, and rabbits is toxic in excessively minute doses, is for the dog harmless in quite large quantities, this is because, the binding haptophore groups being wanting, the ptomaine cannot, in the dog, enter into direct relation with organs essential to life. We see, then, that the haptophore groups act especially in bringing definite areas of the cell within the sphere of influence of the toxophore group. In the behaviour of the haptophore and toxophore groups there exists a difference essentially great, as we have already pointed out when referring to the work of Donitz and Heymans. The haptophore group exercises its activity immediately after injection into the organism, while in all toxines—with the, perhaps,solitary exception of snake-venom—the toxophore group comes into activity after the lapse of a longer or shorter incubation period, which may, e.g.,in the case of diphtheria toxine, extend to several weeks. It is in the highest degree interesting that it is possible, by voluntarily influencing certain of the outside conditions, to exclude absolutely the action of the toxophore group. Courmont has shown that frogs, when kept at a temperature lower than 20° C., manifest no sign of tetanus, even after very large doses of tetanus toxine, but they succumb to fatal tetanus if they are placed in surroundings of a higher temperature. Dr. Morgenroth, working in my Institute, has thrown light on this behaviour by proving that in frogs maintained in cold surroundings the tetanus toxine is fixed in their central nervous system, and that the absence of action at lower temperatures can only be explained by the toxophore group of tetanus toxine having its action restricted within a certain temperature minimum, while independent of this the haptophore group exercises its action on the nervous system at all temperatures.

The theory above developed allows of an easy and natural explanation of the origin of antitoxines. In keeping with what has already been said, the first stage in the toxic action must be regarded as being the union of the toxine by means of its haptophore group to certain “side-chains” of the cell protoplasm. This union is, as animal experiments with a great number of toxines show, a firm and enduring one. The side-chain involved, so long as the union lasts, cannot exercise its normal physiological nutritive function—the taking up of definite food-stuff’s. It is as it were shut out from participating, in the physiological sense, in the life of the cell. We are therefore now concerned with a defect which, according to the principles so ably worked out by Professor Carl Weigert, is repaired by regeneration. These principles, in fact, constitute the leading conception in my theory. If, after union has taken place, new quantities of toxine are administered at suitable intervals and in suitable quantities, the side-chains, which have been reproduced by the regenerative process, are taken up anew into union with the toxine, and so again the process of regeneration gives rise to the formation of fresh side-chains. In the course of the progress of typical systematic immunisation, as this is practised in the case of diphtheria and tetanus toxine especially, the cells become, so to say, educated or trained to reproduce the necessary side-chains in ever-increasing quantity. As Weigert has confirmed by many examples, this, however, does not take place as a simple replacement of the defect; the compensation proceeds far beyond the necessary limit; indeed, over-compensation is the rule. Thus the lasting and ever-increasing regeneration must finally reach a stage at which such an excess of side-chains is produced that, to use a trivial expression, the side-chains are present in too great a quantity for the cell to carry, and are, after the manner of a secretion, handed oyer as needless ballast to the blood.

Regarded in accordance with this conception, the antitoxines represent nothing more than side-chains reproduced in excess during regeneration, and therefore pushed off from the protoplasm; and so coming to exist in a free state. With this explanation the phenomena of antitoxine formation lose all their strange, one might say miraculous, characters. I have deemed it advisable to represent by means of some purely arbitrary diagrams (Plates 6 and 7) the views I have expressed regarding the relations of the cell considered in the manner I have been describing. Needless to say, these diagrams must be regarded quite apart from all morphological considerations, and as being merely a piictorial method of presenting my views on cellular metabolism, and the method of toxine action and antitoxine formation during the process of immunisation.

In the first place our theory affords an explanation of the specific nature of the antitoxines, that tetanus antitoxine is only caused to be produced by tetanus toxine, and diphtheria antitoxine through diphtheria toxine. This very specific nature of the affinity between toxine and cell is the necessary preliminary and cause of the toxicity itself. Further, our theory makes it easy to understand the long-lasting character of the immunity produced by one or several administrations of toxine, and also the fact that the organism reacts to relatively small quantities of toxine by the production of very much greater quantities of antitoxine. By the act of immunisation, certain cells of the organism become converted into cells “secreting” antitoxine at the same rate as this is excreted. New quantities of antitoxine are constantly produced, and so throughout a long period the antitoxine content of the serum remains nearly constant. The secretory nature of the formation of antitoxines has been very strikingly illustrated by the beautiful experiments of Salmonson and Madsen, who have shown that pilocarpine, which augments the secretion of most glands, also occasions in immunised animals a rapid increase’ in the antitoxine content of the serum.

The production of antitoxines must, in keeping with our theory, be regarded as a function of the haptophore group of the toxine, and it is therefore easy to understand why, out of the great number of alkaloids, none are in a position to cause the production of antitoxines. Conversely, indeed, I recognise in this incapacity of the alkaloids, in opposition to the toxines, to produce antitoxines, a further and salient proof of the truth of the deduction I have previously based on chemical grounds, that the alkaloids possess no haptophore group which
anchors them to the cells of organs. To formulate a general statement, the capacity of a body to cause the production of antitoxine stands in inseparable connection with the presence of a haptophore atomic group. In the formation of antitoxine the toxophore group of the toxine molecule is, on the contrary, of absolutely no moment. But the toxoid modifications of the toxines, in which the haptophore group of the toxine is retained, while its toxophore group has ceased to be active, possess the property of producing antitoxines.

Indeed, in some cases of extremely susceptible animals, immunity can only be attained by means of the toxoids, and not by the too strongly acting toxines. The toxoids are certainly able to cause the production of antitoxines. To quote an example, it is hardly possible in an animal, which, like the guinea-pig, has all the tetanophile groups confined to the cells of the central nervous system, to produce immunity by means of the unaltered tetanus toxine, whereas this is attained with extraordinary rapidity and ease by means of its toxoids.

The symptoms of illness due to the action of the toxophore group, therefore, play no part in the production of antitoxine. On the contrary, we may consider that the severe symptoms, which indicate injury to the cell-life, disturb the regenerative functions, and thus hinder or entirely frustrate the course of the immunisation process. I have from the first adopted this view, and it was simply a misunderstanding when Knorr, who has been all too soon taken from the field of his labours, affirmed that, according to my theory, sickness of the cell constituted the necessary condition precedent to the new formation and pushing-off of side-chains.

If I am not altogether deceived, the toxoids, where it is a question of producing an active immunisation (and this will always be the case when the immunisation concerns human beings), are destined to play an important role in practical medicine.

In their theoretical relations the toxoids are also of far-reaching interest, in that they provide a transition to that immunisation which can be called forth by substances which would a priori be considered entirely devoid of toxic character, and which are sometimes, like the autochthonous ferments (i.e., those normally present in blood), products of normal cell-life, and in some cases food-stuffs proper. Thus Dr. Morgenroth, working in my laboratory, has proved that the rennet ferment, if introduced in great quantities into the organism, behaves exactly like a real toxine, in that it causes the production of a typical anti-rennet, which up to a certain limit accumulates in proportionally greater quantity, the greater the injected doses. Here, however, we have to do with processes which are altogether within the region of the normal, as is most clearly shown in certain animals, the horse, in the blood serum of which there is normally present a quantity of anti-rennet, equal to that attained in the goat only after a systematic immunisation carried on for months. The rennet ferment present naturally in the body of the horse is the cause of this great formation of anti-rennet. According to Bordet’s experiments, if injections of milk be given to animals, their serum acquires thereby the capacity to cause flocculent curdling. This action is seemingly rigidly specific because (according to Morgenroth’s experiments) the body produced by the injection of goat’s milk, coagulated goat’s milk, but not human or cow’s milk.

The behaviour is also similar when different kinds of albumin, e.g., the sera of different animals or the white of egg, are injected. There appear constantly in the serum of the animal so treated new substances—specific coagulines—which act only in a specific manner, i.e., precipitate only the form of albumin injected. Thus there are produced, by the injection of common food-stuffs, typical “Antikorper,” which unite with the substances used to occasion their production, and form with them insoluble combinations.

From all these considerations I think myself warranted in concluding that the formation of antitoxines lacks all the characters of that purposeful, intelligently directed, and remarkable process which it at first seemed to be, and that it is to be regarded merely as a process analogous to those constituting an essential portion of the normal metabolism of the organism. We must admit that the majority of the food-stuffs and of the intermediate products of tissue-change must be able to cause the production and throwing-off of nutritive side-chains. It may be that the new formation only takes place to a limited extent, and that the replacement of any side-chains which have been shut out from their physiological function is all that is accomplished; but the formation may occur in greater proportions, may become excessive, and therefore lead to the presence of “Antikorper” in the blood.

In this way is easily explained the fact of the occurrence in the normal blood serum of antitoxines and of bodies inimical to bacteria, without the animals having ever been brought into relation with the corresponding toxines or bacteria. Here I need only refer to the fact that diphtheria antitoxine is not uncommonly present in normal horses and in men who have never suffered from diphtheria. Particularly weighty in this connection are the observations that have been made on horses, because, on the one hand, these animals never suffer from diphtheria, and, on the other hand, Cobbett has brought forward experimental proof that this normally occurring antitoxine corresponds absolutely as to its properties with the antitoxine produced by artificially immunising. The conclusion, therefore, is that in the body of the normal horse certain substances may be present which possess side-chain affinities similar to those of the diphtheria toxine, and which, therefore, are quite as capable as the latter are of taking possession of the cell side-chains, and occasioning the regeneration and pushing off of these from the cells; in other words, of causing the presence of an actual diphtheria antitoxine in a normal animal.

Such occurrences direct attention to the possibility of producing immunity in some cases by the administration of definite food-stuff’s. Perhaps we have in some such peculiarity of feeding and tissue-change the explanation of the fact so difficult to understand, viz., that individuals of the same race and species react in such diverse manners to the same infection. Certainly we are very far removed from the solution of this important question, which, as yet, has scarcely assumed a tangible form. Still it is our duty to strive with tenacity to overcome the difficulties which surround this point, bearing in mind the words of your illustrious countryman, Francis Bacon: “Sunt certe ignavi regionum exploratores, qui, ubi nil nisi coelum et pontus videtur, terras ultra esse prorsus negant.”

https://doi.org/10.1098/rspl.1899.0121

In Summary:

• Ehrlich praised the work of Edward Jenner and his smallpox inoculations but wondered why no progress had been made with his findings for over 100 years to induce an artificial immunity in the case of other infectious diseases
• Jenner’s work remained isolated due to there being no theoretical explanation for the cause and nature of infectious disease
• Ehrlich gave credit to Pasteur and Koch for allowing the work on artificial immunization to continue
• The artificial immunity could be done by passaging through an animal body or through artificial culture methods
• The theoretical explanations of all these facts lagged far behind their practical effects
• Ehrlich credited Behring for the “discovery” of unseen bodies in the blood creating immunity against diphtheria/tetanus but he was disappointed by the lack of progress in regards to expanding Behring’s research
• It was disappointing when there did not follow, on the “successful” practical application of diphtheria antitoxic serum, a rapid succession of similar achievements
• By purely empirical methods, e.g., by the production and use of sera of very great antitoxic value, the results attained showed no improvement
• Better success was only to be hoped for by an accurate knowledge of the theoretical considerations underlying the question of immunity
• Ehrlich began his theoretical investigation by looking at the toxins
• He stated that the toxines, i.e., the poisonous products of bacteria, were unknown in a pure condition
• So great is their potency, it had to be assumed that the strongest solid poisons which were obtained by precipitating toxic bouillon with ammonium sulphate, represent nothing more than indifferent materials, peptones and the like, to which the specific toxin attaches itself in mere traces beyond the reach of weighing
• Up to that time, by the purely chemical methods of weighing and measuring, it had been impossible to ascertain anything as to their presence or the intensity of their action
• In other words, there was no chemical way to ascertain the presence or intensity of action of toxins
• The presence of the toxin is only betrayed by the proof of their specific toxicity on the organism (i.e. the only proof is the effect as the causal agent can not be seen)
• For the exact determination of the amount of toxin contained in a culture fluid, the essential condition was that the
  research animals used should exhibit the requisite uniformity in their susceptibility to the poison, however, uniformity is not observed in the reaction of the animal body to all toxins
• For many toxins, accuracy in measuring the toxicity cannot be attained
• By means of test-tube experiments with animal blood, Erlich claimed he was able to determine that they yielded an exact quantitative representation of the course of the processes in the living body
• The demonstration of this “fact” formed the basis of a more extended application of experiments of this nature
• The knowledge gained led easily to the inference that to render toxin innocuous by means of antitoxin was a purely chemical process, in which biological processes had no share
• However, insurmountable obstacles presented themselves to this conclusion
• Ehrlich stated that it must be postulated that in chemical processes the bodies sharing m the action react with one another in definite equivalent quantities
• This proposition appeared, however, not to hold in the case of the action of antitoxin on diphtheria toxin
• He tried to establish a unit of measurement through inference by looking at diphtheria but the numbers did not hold
• He attempted to make it so that in every case the law of equivalent proportions would hold good
• Ehrlich openly pondered how one can actually measure the strength of a toxin accurately
• He came to regard as the “toxic unit” that quantity of toxic bouillon which exactly sufficed to kill, in the course of four days, a guinea-pig of 250 grammes weight
• Erhlich created estimated units of measurement: the “simple lethal dose” and the “immunity unit”
• The conclusions led him to determine that neutralization of toxin by antitoxin did not follow the law of equivalent proportions
• He was “obliged to conclude” that the chemical affinity for toxin-antitoxin did not exist due to contradictory results obtained
• The results could only be explained by ASSUMING that there were two different groups that characterized the toxin:
  • Haptophore: conditions the union with antitoxin
  • Toxophore: the cause of the toxic action
• Ehrlich inferred that the toxophore group was unstable whereas the haptophore group was more stable
• He claimed that his conception of the constitution of diphtheria toxin, after more extensive, very exact, and much varied experimentation, based on its partial neutralisation by antitoxine, had been confirmed in the completest manner possible
• However, at that time, he felt it would be superfluous (unnecessary, especially through being more than enough) to enter into all the details pertaining to these investigations
• Ehrlich stated his theory about these two groups allowed him to progress as it provided a satisfactory chemical explanation for neutralization but also the key to the toxic nature of toxins as well as the mystery of the origin of antitoxins
• That chemical substances are only able to exercise an action on the tissue elements with which they are able to establish an intimate chemical relationship is a conception of a general nature, which has been entertained since the birth of scientific medicine
• Ehrlich felt it was astonishing, almost astounding, that this axiom, of which the theoretical importance had been so long recognised, and which served as the first ground for certain therapeutical procedures, should as a matter of fact have played in the building up and furtherance of scientific pharmacology a role so insignificant in proportion to its great importance
• The methods for obtaining any knowledge of the exact distribution of drugs in the body were very imperfect
• The investigation of the laws pertaining to the minute distribution of a chemical substance in the body is only possible when these are at once recognisable by the eye
• He felt that if they are not able to discover the principles governing the localisation of common chemical bodies, which can be used in suitable quantities in chemical purity, and which chemical and other reactions render perceptible, it was a priori very unlikely that efforts directed to locating the toxins, which are potent in the slightest traces, and which are bodies they have no means of rendering perceptible to the senses, would be anything else than absolutely without result
• In other words, Ehrlich was making the case that chemical reactions needed to stand in as evidence for the invisible particles
• He shared evidence that the length of time, by a matter of minutes, determines if an antitoxin will counteract a toxin
• If a certain period of time had elapsed and even if an animal showed no symptoms, not even the greatest amount of antitoxin would save it
• Through these observations with tetanus toxins, Ehrlich was “forced” to accept that the central nervous system has similar atomic bodies as to the antitoxin
• He felt that the predilection of the nervous system for tetanus toxin, the rapid union of the toxin with the nervous tissue, the gradual onset of the symptoms and their long duration could only be explained by the existence of such toxophil groups (i.e. his imaginary creations were the only possibility)
• Ehrlich felt that the fact that the toxicity in guinea pigs being greater when injected subcutaneously rather than intracerebrally could only be explained when injecting toxin directly into the brain, the toxophile atom groups there present at once seize on all the toxin, while when the toxin is administered through the blood stream, the toxophile groups present in other organs also take up the toxin in equivalent quantities
• Ehrlich stated that it can not be entertained that these unseen atmoic bodies exist only to remove toxins and must have other cellular functions
• Accordingly, he felt one may rightly assume that these toxophile protoplasmic groups in reality serve normal functions in the animal organism, and that they only incidentally and by pure chance possess the capacity to anchor themselves to this or that toxin
• The first thought suggested by this assumption was that the atom groups referred to must be concerned in tissue change
• Ehrlich then formulated many assumptions such as:
  1. The protoplasm has certain atomic bodies that fix to food-stuff important to cellular life called side-chains
  2. That they are free and independent to varying degrees and may differ due to requirements by different cells
  3. That every atom-group has a corresponding side-chain, i.e. lock and key
• Ehrlich believed that the mystery of immunity ceases when looking through his view that the haptophore fit with antitoxins/side-chains and they bind the toxin to the cell
• Discussing experiments using tetanus and guinea-pig brains, Ehrlich made the deduction that an “anchoring” took place as an unseen chemical union between the brain substance and the tetanus toxin made it non-toxic with later use
• He stated that the possession of a toxophile group by the cell is the necessary preliminary and cause of the poisonous action of the toxin
• This is demonstrated in the case of certain blood poisons, like the hsemolysines, which exercise a solvent action only on such red blood corpuscles as are able to unite chemically with them
• The union with the red corpuscles can be proved, and one has here the great advantage of dealing with living and intact red blood cells instead of broken-down cellular material (as is the case in virology studies)
• Ehrlich stated that he was obliged to conclude that these toxins are only in a position to act prejudicially on the organism if they are able, by means of their haptophore groups, to anchor themselves to the side-chains of the cells of organs essential to life
• If the cells of these organs lack side-chains fitted to unite with them, the toxophore group cannot become fixed to the cell, which therefore suffers no injury, i.e., the organism is naturally immune
• Ehrlich believed his theory allowed an easy and natural explanation for the origin of antitoxins
• His theory proposed that side-chains regenerate to a point where they break off the cell into the bloodstream
• Ehrlich stated that with his explanation of the phenomena of antitoxin formation, it loses all the strange and miraculous characters
• Ehrlich presented drawings which he admits are not morphologically accurate but are a representation of his thoughts on how immunity occurs
• He claimed his theory affords an explanation of the specific nature of the antitoxins
• He also felt his theory made it easy to understand the long-lasting character of the immunity produced by one or several administrations of toxin
• In order to keep with his theory, Ehrlich stated that the production of antitoxins must be regarded as a function of the haptophore group of the toxin
• To formulate a general statement, the capacity of a body to cause the production of antitoxin stands in inseparable connection with the presence of a haptophore atomic group (i.e. the body can not make antibodies without the antigen)
• In some cases of extremely susceptible animals, immunity can only be attained by means of the toxoids, (i.e. vaccination) and not by the too strongly acting toxins (i.e. natural immunity)
• Ehrlich admitted that SEVERE SYMPTOMS associated with immunization is against the very nature of immunity and hinders/disrupts the process
• He said he adopted this view from the start, and it was simply a misunderstanding when Knorr affirmed that according to Ehrlich’s theory, sickness of the cell constituted the necessary condition precedent to the new formation and pushing-off of side-chains
• He stated that if he is not altogether deceived, the toxoids (i.e. antigens), where it is a question of producing an active immunisation, are destined to play an important role in practical medicine
• The theoretical relations of the toxoids were also of far-reaching interest to Ehrlich
• He claimed that “Antikorper” (a.k.a. antibodies) were produced by the injection of common food-stuffs, which unite with the substances used to occasion their production, and form with them insoluble combinations
• Ehrlich felt that they must admit that the majority of the food-stuffs and of the intermediate products of tissue-change must be able to cause the production and throwing-off of nutritive side-chains
• He referred to the fact that diphtheria antitoxin is not uncommonly present in normal horses and in men who have never suffered from diphtheria
• The conclusion was that in the body of the normal horse certain substances may be present which possess side-chain affinities similar to those of the diphtheria toxin, and which, therefore, are quite as capable as the latter are of taking possession of the cell side-chains, and occasioning the regeneration and pushing off of these from the cells; in other words, of causing the presence of an actual diphtheria antitoxin in a normal animal
• In other words, horses and men with no exposure to diphtheria had antibodies to diphtheria similar to if they had been vaccinated

As can be see from this section of the lengthy highlights of Ehrlich’s report, he did nothing more than create a story around a possible way immunity occurs inside the body. Nothing he said was ever physically seen nor proven by his theoretical investigation. It is all conjecture and inference which was later taken to be fact due to further indirect evidence. Ehrlich used his lively imagination to conjure up a narrative for how immunity works which became the very foundation for all of Immunology today. It does not matter that his theory was based on nothing but chemical reactions and invisible entities nor that his theory turned out to be less than accurate. Ehrlich established the bottom layer of this house of cards known as Immunology. Like virology, it is a house of cards ready and willing to come tumbling on down.