CHAPTER XI

NATURAL IMMUNITY AGAINST TOXINS

Examples of natural immunity against toxins.—Immunity of spiders and scorpions against tetanus toxin.—Immunity of the scorpion against its own poison.—Antivenomous property of the blood of the scorpion.—Immunity against tetanus toxin in the larvae of _Oryctes_ and in the cricket.—Immunity and susceptibility of frogs against this toxin.—Natural immunity of reptiles against tetanus toxin.—Antitetanic property of the blood of alligators.—Immunity of snakes against snake venom.—Immunity of the fowl against tetanus toxin.—Immunity of the hedgehog against poisons and venoms.—Immunity of the rat against diphtheria toxin.

[Sidenote: [341]]

As in this book we are dealing specially with the immunity against infective diseases, the question of the resistance of the animal to poisons interests us only in so far as it is related to immunity against micro-organisms. Consequently the reader must not expect a treatise on intoxications properly so called nor one on immunity against all kinds of poisons. To perform such a task we should have to far overstep the bounds of the subject that we have chosen and enter upon an examination of questions which are beyond our sphere. Our chief aim is to present to the reader a summary of our present knowledge on immunity against microbial toxins and to establish the relations between this kind of immunity and immunity against infective micro-organisms. In order to do this, however, we shall have now and again to go beyond the limits of our programme and discuss certain problems bearing on the resistance of the animal organism against poisons not of microbial origin.

[Sidenote: [342]]

The immunity against toxins, like that against the micro-organisms themselves, may be either natural or acquired. As many poisons have been known from time immemorial, we are able to collect numerous observations on the resistance of the animal organism to such substances made when there was no idea of immunity against infective diseases. The etiology of intoxications is often much more evident and simple than is that of infections; this is one of the reasons that the older conceptions on the subject of immunity against poisons were more advanced than were those on immunity against infective diseases.

Several examples of natural immunity in the lower animals have already been cited. Thus, we have seen in previous chapters that the Infusoria are resistant to poisons that exert a powerful action on a large number of the higher animals, such as the tetanus and diphtheria toxins and especially the ichthyotoxin of eel’s serum. We have mentioned the case of the larva of _Oryctes nasicornis_ which is unaffected by large doses of the toxins of certain bacteria and which at the same time is very subject to fatal infections by very small doses of the bacteria that form the poisons. These larvae, like those of the cockchafer, are, however, fairly susceptible to the poison of the scorpion. Several other species of Arthropoda, which have been studied from the point of view of immunity against toxins, have exhibited analogous features. Thus spiders and scorpions are refractory to tetanus toxin. In one experiment I injected into the abdominal cavity of a _Mygale_ from the Congo (which weighed 7 grm. 5) 1 c.c. of tetanus toxin on two several occasions. This dose is sufficient to kill, with the symptoms of tetanus, 1000 mice of double the weight. The spider, kept in the incubator at 36° C., remained quite well during the two months that the experiment lasted. It exhibited no symptom, not even transient, of muscular stiffening, nor any change in its habits and natural functions. The tetanus toxin disappeared from the blood of the _Mygale_, but this blood at no time showed the slightest antitoxic power against this poison. This example of natural immunity cannot, therefore, be ascribed to any antitoxic property of the fluids and must be regarded as a case of immunity of the tissues—von Behring’s histogenic immunity. In the present imperfect state of our knowledge it is impossible to describe precisely the mechanism of this immunity. When we say that the spider is refractory to the tetanus toxin because its susceptible elements have no receptors capable of seizing the haptophore group of this poison, we simply give expression to a hypothesis which we are not in a position to verify by experiment.

[Sidenote: [343]]

The scorpion, a well-known representative of the Arachnida with segmented abdomen, shares with the _Mygale_ in the immunity against tetanus toxin. The Algerian and Tunisian scorpions (_Scorpio afer_ and _Androctonus occitanus_) withstand the action of doses of this poison which are fatal for 1000 mice and more. Taking weight as our standard we may inject into them, with impunity, more than 5000 times as much toxin as into mice, without setting up a single morbid symptom. Scorpions, like the _Mygale_, live well in the incubator at 36° C., where they are kept whilst submitted to the action of the tetanus poison. Here again we have to do with a case of histogenic immunity. The fluids of the scorpion exert no antitoxic action. When blood from the normal scorpion is mixed with various doses of tetanus toxin and injected into mice these animals contract tetanus and die just as do the control animals. In certain exceptional cases some slight retardation was observed, but the blood of the scorpion is, in most cases, incapable of preventing tetanus in animals susceptible to this disease.

Scorpions, injected with tetanus toxin, do not retain it in their blood for long. A few days after the injection of the tetanus poison such blood, when injected subcutaneously into mice, excites no trace of tetanus. The preparation of extracts of the different organs of scorpions treated with tetanus toxin demonstrates that the liver and the liver only absorbs the poison. It is found there a few days after the injection of the toxin and it remains there unaltered for some considerable time. The exudation of the liver of scorpions, killed a month or more after the introduction of the toxin into the general cavity, injected into mice sets up a typical and fatal tetanus.

The presence of the tetanus toxin in the organism of scorpions does not give rise to the production of antitoxin. At any rate a whole series of experiments on this point carried out by us never gave a positive result. The scorpions resisted repeated doses of the tetanus toxin and lived without any difficulty at 36° C., but their blood was never at any period capable of preventing mice from contracting fatal tetanus. Nevertheless the scorpion may possess antitoxic power.

[Sidenote: [344]]

Everyone has heard of the supposed suicide of the scorpion. We are told that when this animal finds itself under conditions in which its death is inevitable, it stings itself with the end of its tail and dies from the effect of its own poison. A simple method of reproducing this experiment is actually described:—Surround the scorpion with a circle of fire. The animal rushes in all directions to find a way out, and finding none, deliberately commits suicide. Bourne[501] at Madras carefully investigated this question in a large species of Indian scorpion and demonstrated the absolute erroneousness of the story of suicide which, had it been true, would have afforded a unique example of voluntary death in animals. On carrying out the classic experiment he observed that within this ring of fire the scorpion is subjected to a very high temperature. When the temperature reaches 40° C. the scorpion begins to grow weak and as the temperature approaches 50° C. it passes into a comatose condition. Moreover Bourne showed that the scorpion’s poison, which is fatal for large spiders, insects, and vertebrates, was innocuous for individuals of the species furnishing it.

I can confirm all the statements of this English observer. When I was studying the embryology of the scorpion I repeatedly tried the experiment but the animal never committed suicide. Further, I repeatedly assured myself of the innocuousness of the scorpion’s poison when injected into individuals of the same species, and I was able to demonstrate most conclusively that the blood of the scorpion is endowed with undoubted antitoxic power. The addition of 0·1 c.c. of this blood to a dose of poison which kills mice in half-an-hour is sufficient to enable a mouse injected with the mixture to resist it completely. This antitoxic power is the same in the _Scorpio afer_ and in the Algerian _Androctonus_. An emulsion of the liver of the scorpion, however, is absolutely incapable of preventing fatal intoxication of mice.

[Sidenote: [345]]

This case of antitoxic action is the only one I have been able to demonstrate in an invertebrate. Must we regard it as a case of natural innate antivenomous power or as something acquired during the life of the animal? It is not easy to settle this question by experiment. We can certainly procure new-born scorpions and rear them for some time, but the quantity of blood that can be got from them is insufficient for injection for protective purposes. Scorpions do not love one another and when kept together we often find them engaged in fierce and mortal combat, the stronger killing the weaker and sucking their blood. It is therefore possible that, in some stage of their life, the scorpions find means of vaccinating themselves against their own poison either through the intestine or as the result of punctures caused by the point of the tail. It would be very interesting to study this question under favourable conditions, because it is capable of throwing light on the problem of the origin of antitoxins, from a general point of view. Whichever view be taken, the acquisition of any antitoxic property by the blood in the Invertebrata must take place slowly and with great difficulty as is shown by our want of success with tetanus toxin.

Insects are, as a rule, very tolerant of this latter poison. As, however, the tetanus toxin (we shall illustrate this later) only acts well and in small doses at a high temperature (about 30° C.) and as most insects do not readily adapt themselves to this temperature, it was necessary to choose species capable of living at these high temperatures and for this line of study the larva of _Oryctes_ is most suited. It flourishes well at a temperature of 30°–36° C., and under these conditions exhibits a much greater resistance to infection by _Isaria_ than at lower temperatures. It can be kept in the incubator for months if placed in glass jars filled with earth mixed with tanner’s bark. The injection of enormous quantities of very active tetanus toxin directly into the blood has not the slightest effect on these larvae. Whilst, however, the blood fluid of the Arachnida rapidly gets rid of the poison, that of _Oryctes_ retains it for a very long period. If a small quantity of blood be taken from larvae several months after injection and then injected into mice, these animals contract typical tetanus and quickly succumb.

The toxin, however, finally disappears from the blood though a certain portion of it may still be found in the pericardial cells and especially in the fat-bodies.

Never, under any circumstances, was I able to observe that the blood of the larvae of _Oryctes_ exerted any antitoxic action. At the stage when this fluid no longer gives tetanus to mice, it is absolutely incapable of preventing intoxication when mixed, before injection, with tetanus toxin.

Amongst adult insects the cricket is best adapted for researches on tetanus. The field cricket will bear a temperature even higher than 30° C. It is completely resistant to injections of tetanus toxin, but it showed no more capacity than did the larvae of _Oryctes_ or the Arachnida of producing any tetanus antitoxin.

[Sidenote: [346]]

All the Invertebrata that I have been able to study have exhibited a remarkable resistance against the known bacterial toxins, but the mechanism of this natural immunity could not be exactly made out owing to the difficulty met with in investigating the toxins in the organs and following their modifications. The idea of making use of these lower animals for the purpose of solving the problem of the origin of antitoxins is not realisable, from the fact that the Invertebrata that have been studied have never, in my experience, produced any of these substances as the result of injections, whether single or repeated, of toxins.

The natural immunity of the Invertebrata against bacterial toxins cannot therefore be regarded as an example of humoral immunity. It must be placed in the category of histogenic immunity, although we are not in a position to define accurately the part played by the cellular elements in the defence of the animal against these poisons. We must, therefore, go higher up in the animal scale if we are to solve the principal questions in regard to antitoxic immunity.

[Sidenote: [347]]

The lowest Vertebrata, the fishes, are not well-suited for this kind of research. The best known bacterial toxins act specially on warm-blooded animals and require the co-operation of high temperatures. Fishes do not live well in captivity except at relatively low temperatures and soon die if placed in an incubator kept at 30° C. or higher. It is necessary, therefore, to have recourse to the Amphibia, which are much more easily acclimatised to these temperatures. The Axolotl, coming from Mexico, is naturally capable of withstanding great heat. These animals will live for long at a temperature of 30°–37° C. They possess the drawback, however, of being very susceptible to the tetanus toxin, very small doses of it being fatal. The green frog (_Rana esculenta_) is the most suitable for our purpose. It readily adapts itself to optimum temperatures (30°–36° C.) and exhibits at least a certain degree of immunity against various bacterial toxins. We have stated in a preceding chapter that the green frog is unaffected by considerable quantities of diphtheria toxin. It is resistant also to tetanus toxin, but this natural immunity appears to be connected with special conditions. Courmont and Doyon[502] were the first to draw attention to the fact that beyond 20°–25° C. green frogs may contract tetanus. Refractory in winter they become susceptible in summer. These observers afterwards found that of frogs inoculated with the same dose of toxin and divided into two sets, one set kept at a temperature of about 10° C. remained quite well whilst the other set subjected to one of 30°–39° C. contracted tetanus after five days’ incubation. This experiment has been confirmed by several observers, and indicates that the tetanus poison demands, for the manifestation of its toxic action, a favourable and fairly high temperature. This result must, however, be accepted with some reserve. Undoubtedly the doses of tetanus toxin which induce fatal tetanus in frogs kept at a high temperature are innocuous when these animals are living at low temperatures. But we can, by increasing the dose, produce tetanus in frogs even when the temperature is not very high. Thus Marie[503] was able, during the whole of the winter, to tetanise both green and brown frogs living in water the temperature of which oscillated between 13° and 18° C. The incubation period in this case is very much longer (sometimes extending to 25 days) than in frogs kept at higher temperatures.

Temperature, therefore, is an important factor in the poisoning by the tetanus toxin and in the resistance of the frog, but, in the long run, this poison can exert its specific action even at relatively low temperatures.

[Sidenote: [348]]

Morgenroth[504] endeavoured to analyse the mechanism of this resistance and of the susceptibility of the green frog when maintained at various temperatures. He demonstrated that the tetanus toxin is fixed in the central nervous system, even at low temperatures, near 8° C.; under these conditions, however, it is incapable of causing the slightest tetanic symptom. When placed in an incubator kept at 32° C. the frogs contract tetanus after a period of incubation of some (2 to 3) days. During the first 24 hours of this period the frogs manifest no sign of tetanus, and if they are again put in a cool place they continue in good health. If, however, after a not too prolonged stay in the cold, these animals are subjected a second time to the higher temperature, they become tetanic, after a shortened incubation period. Cold, therefore, may arrest tetanus even at a stage when the toxin has already produced certain latent but permanent modifications of the nervous system.

Frogs injected with tetanus toxin and kept in a cold place finally get rid of the poison. When transferred to a warm chamber after a certain lapse of time they no longer contract tetanus. We have found that the greater part of the tetanus toxin continues for some time in the blood of frogs injected and kept at a low temperature. A small quantity of this blood withdrawn almost two months after the last injection produced fatal tetanus in a mouse. We do not know how frogs eliminate the toxin, but it has been demonstrated that in this case it causes no production of antitoxin. Morgenroth has confirmed this result.

[Sidenote: [349]]

Reptiles must be regarded as vertebrates exhibiting a most pronounced natural immunity against tetanus. They show an unlimited resistance to enormous doses of tetanus poison, and this at low, medium, or high temperatures (30°–37° C.). Green lizards withstand considerable doses of tetanus toxin. Although they do not contract tetanus, they get rid of the poison exceedingly slowly. Thus, a lizard kept at a temperature of 20° C., and injected with an amount of toxin sufficient to kill 500 mice, at the end of two months still retains in its blood such an amount of the poison that one-tenth of a c.c. will cause fatal tetanus in a mouse. Turtles present an analogous case. The marsh turtle, _Emys orbicularis_, tolerates very large amounts of tetanus toxin, injected subcutaneously, and this at both low and high temperatures, at 30° C. and beyond (36°–37° C.). The toxin passes quickly into the blood and remains localised there for a very long time. In a turtle kept in an aquarium at the laboratory the blood was tetanigenic for the mouse even four months after an intraperitoneal injection of the toxin. In another turtle which lived at incubator temperature (36°–37° C.), the blood was still toxic two months after a subcutaneous injection of tetanus toxin in quantity fatal for 500 mice. In turtles kept at 36° C. I observed abundant transudations into the peritoneal cavity, and the fluid, very poor in formed elements, was found to be very tetanigenic. It must be accepted, therefore, that the toxin is retained in the blood plasma with which it passes into the transudation. Every kind of cell must exhibit a very marked negative chemiotaxis against tetanus toxin for this poison to be retained so long in the body fluids. Under these conditions it is not surprising that in turtles I was never able to observe the slightest antitoxic power in the blood. Their great natural immunity must be due to some other factor.

The alligator (_Alligator mississipiensis_) has also been found to be quite refractory to tetanus both at low and at high temperatures. Outwardly alligators behave exactly as do turtles, that is to say, after the injection of various and sometimes very large doses of toxin they exhibit no morbid symptom either general or tetanic. But the particular changes which occur in their organism differ essentially from those met with in the turtle. The toxin is rapidly eliminated from the blood of the alligator, even when it is kept at a relatively low temperature (20° C.). Under these conditions of temperature, however, the blood does not become antitoxic although it has lost its tetanigenic property. When, however, the alligators are kept at a higher temperature (32°–37° C.), antitoxic power is developed in their blood, often with very great rapidity. Quite young alligators (weighing about 500 grammes) are capable of producing antitoxin, though somewhat slowly. A month after the first injection of the tetanus toxin their blood is incapable of causing tetanus in mice, but is not yet antitoxic. A month later, however, it never fails to prevent an attack of tetanus when mixed with fatal doses of the toxin and injected into mice.

Older alligators develop antitoxic power much more rapidly, and on several occasions we have found, to our great astonishment, that, as early as 24 hours after injection of the toxin, their blood was distinctly antitetanic. The blood of the same alligators, tested before the injection of the toxin, like the blood of normal alligators generally, exhibited no antitoxic property.

In several experiments we took the rectal temperature of our animals and were never able to observe the slightest rise corresponding to the temperature of the water in which the alligators lived.

[Sidenote: [350]]

It cannot be doubted then, that, in spite of the facility with which these reptiles produce tetanus antitoxin, their immunity does not depend on this antitoxic property. Thus, young alligators which have resisted a single dose of toxin sufficient to kill 6000 mice must owe their immunity to some other cause than the antitoxic power of the body fluids, for their blood does not begin to exhibit this property until two months after injection.

These same reptiles are also very refractory against cholera toxin, even in large doses; they react to the injection by the development of the corresponding antitoxin. On the other hand they are very susceptible to diphtheria toxin, small quantities of which are quite sufficient to bring about a fatal intoxication.

Snakes, like other reptiles, are refractory against tetanus toxin. In the study of their natural immunity, however, we are confronted by the difficulty that their blood is naturally toxic for laboratory animals. This toxin, analogous to the ichthyotoxin of eel’s serum, has been compared with snake venom against which the snakes themselves enjoy a very marked immunity.

[Sidenote: [351]]

Not venomous snakes only exhibit immunity against their own poison. Long ago Fontana[505] observed that non-venomous snakes resist the bite of the viper and even subcutaneous inoculation of its venom. Phisalix and Bertrand[506] confirmed these observations and were able to show that a non-venomous snake (_Tropidonotus_) will withstand a dose of venom capable of killing from 15 to 20 guinea-pigs. Seeking for the cause of this natural immunity, these observers came to the conclusion that it is due to the presence in the blood of toxic substances analogous to those of the venom of the viper. These same substances are found also in the labial glands of the upper jaw of the _Tropidonotus_ and can from thence, according to the view of Phisalix and Bertrand, pass into the blood as an internal secretion. Calmette[507] has shown that the blood of snakes, injected in a non-toxic dose, vaccinates certain mammals against snake venom, and Phisalix and Bertrand have even obtained an antitoxic effect by injecting a mixture of snake’s blood, heated to 58° C., with lethal doses of venom. There is, then, in this example something analogous to what we have described in scorpions, with this difference, however, that the blood of these Arachnids is already antitoxic, to a certain degree, whilst that of snakes only becomes so after it has been modified by heat.

The classic example of immunity against a bacterial toxin amongst Birds is that of the fowl, which is highly refractory against the tetanus toxin. In the very earliest researches on this poison injections were made into vertebrates of very different kinds, and a very striking feature was the facility with which fowls resist very large quantities of tetanus toxin. However, as is almost always the case, this immunity has been found not to be absolute. By means of enormous doses, injected subcutaneously or into the muscular tissue, tetanus of the most typical kind, ending in death, has been induced in fowls, and in fowls weakened by cold, tetanic intoxication, even with smaller doses, has been set up. By injecting the toxin directly into the brain, according to Roux and Borrel’s method, the fowl may be still more easily tetanised. Thus, von Behring[508] observed that by injecting one milligramme of the toxin into the brain of a fowl, weighing one kilo, tetanus may infallibly be produced.

After the brilliant and fruitful discovery of the antitoxic property of the blood, made by von Behring in collaboration with Kitasato, we were justified in concluding that immunity against toxins and, amongst others, natural immunity, might depend on the power of the body fluids to neutralise the toxins. This hypothesis has been formulated at various times, but it was for the first time subjected to experimental control by Vaillard[509], and specially in connection with tetanus in the fowl. The blood or blood serum of these birds, when mixed in varying doses, small, medium, and large, with tetanus toxin, was never found to be capable of preventing susceptible animals (mice, guinea-pigs, rabbits) from contracting tetanus: these animals so treated behaved just as did the controls inoculated with toxin only.

[Sidenote: [352]]

The great resistance of the fowl against tetanus,—one of the most typical examples of natural immunity against a microbial poison,—cannot, therefore, be explained by the presence in the body fluids of an antitoxin capable of neutralising and rendering innocuous the tetanus toxin. On the other hand, we are not justified in attributing it simply to the absence of corresponding receptors in the sensitive nerve cells. Since the fowl readily contracts tetanus when the toxin is injected directly into the brain or when the fowl is weakened by cold, it is evident that the sensitive elements never fail to absorb and fix any poison that is presented to them. In ordinary cases, however, when the fowl exhibits its remarkable resisting power against the toxin injected in very large quantity, subcutaneously, into the muscles or into the peritoneal cavity, the poison does not reach the sensitive cells, being arrested and rendered innocuous whilst circulating in the tissues of the organism.

Von Behring[510] is of opinion that in examples of natural immunity, such as the one just examined, the principal cause of the refractory condition depends upon the impermeability to the toxin of the capillary wall of the vessels. It is, however, difficult to maintain this thesis in regard to tetanus in the fowl, when it is remembered how readily tetanus toxin passes through filters and membranes, and especially in view of the fact that weakening of the fowl by means of cold renders it susceptible to doses of toxin which are tolerated without inconvenience by normal fowls.

We are, therefore, compelled to place the natural immunity of the fowl against tetanus toxin in the category of cell immunities. This toxin, as we have said, must be arrested _en route_ before it reaches the cells of the nerve centres. But where and how does this beneficent arrest take place? Ten years ago Vaillard demonstrated that the blood of fowls that have received an injection of tetanus toxin causes typical tetanus in susceptible animals. This tetanigenic property of the blood persists for a certain number of days. When it is measured by the quantitative method, it is found that all or almost all the tetanus toxin injected into the peritoneal cavity of the fowl passes into the blood and remains there intact for a variable number of days. From a morphological point of view the blood, immediately after the injection of the toxin, exhibits a hyperleucocytosis of greater or less duration.

[Sidenote: [353]]

When the fowls are killed at the stage when their blood becomes tetanigenic (as the result of the injection of the toxin into the peritoneal cavity), it can be demonstrated that their viscera are not capable of producing tetanus in susceptible animals except in so far as they contain blood. It is only the vascular organs, rich in blood, such as the spleen, liver, kidneys, thyroid gland and bone-marrow, that impart tetanus and then only in so far as they have not been freed from blood. Of the various organs only the genital glands, ovaries or testes, absorb a certain amount of the injected toxin. Very young testes or the smallest ovarian ova containing as yet no trace of yellow yolk, when injected into mice, produce a fatal tetanus.

In fowls, insusceptible to tetanus toxin, this toxin is found, then, in the sexual glands and in the blood. When, in order to ascertain the exact localisation of this toxin, we measure the tetanigenic power of the whole blood as compared with that of the aseptic exudations induced by the injection of gluten-casein, and necessarily much richer in leucocytes, we get the result that the exudations contain more tetanus toxin than does the blood. We are led, therefore, to the conclusion that this poison is absorbed, at least in part, by the leucocytes, and it is in these elements and in the genital cells that we must look for the factors which arrest the toxin and prevent its reaching the nerve centres.

Cellular or histogenic immunity is often contrasted with chemical immunity without taking into consideration the real analogies and differences to be found between them. It is evident that in both groups the organism of the animal modifies the introduced toxins and that this modification is a chemical process. In cellular immunity, however, this act is preceded by certain biological phenomena, such as the reaction of the formed elements and the absorption of the noxious substance. Immunity in these cases is more complex than in the example where the toxin is neutralised by a direct action of the body fluids, but ultimately it always resolves itself into a chemical or perhaps physico-chemical action of the substances of the organism of the animal on the toxic substances of the poisons.

[Sidenote: [354]]

In Mammals examples of natural immunity against certain poisons are not rare. Almost a century ago Oken made the observation that a person who tried to poison a hedgehog with opium, hydrocyanic acid, arsenic or mercury bichloride usually failed in his attempts because of the great resisting power of this animal. Harnack demonstrated that the hedgehog will withstand a dose of potassium cyanide six times as great as that necessary to kill a cat in a few minutes (0·01 grm.). In Lewin’s[511] experiments the hedgehog was found to resist the injection of powdered cantharides in a quantity seven times as great as that which infallibly kills a dog and greater also than the lethal dose for man. The same observer also confirms the observation that a much larger dose of alcohol must be used in order to intoxicate a hedgehog than is required to obtain the same effect in the rabbit or even in the dog. Horvath[512] fed hedgehogs for a fairly long period with living cantharides. These Insectivora devour their venomous prey without showing any sign of illness except a certain degree of emaciation. When Lewin tried to ascertain the cause of this natural immunity of the hedgehog he examined the blood of this animal for a substance antitoxic to cantharidine. His experiments were all negative; but it is difficult to come to any definite conclusion in this matter from the fact that the blood and blood serum of the normal hedgehog are toxic for the small laboratory animals. A similar objection had already been brought forward by Phisalix and Bertrand in connection with their experiments, analogous to those of Lewin, on the immunity of the hedgehog against the venom of the viper.

[Sidenote: [355]]

It has long been known that hedgehogs have a liking for certain reptiles and wage an implacable war on snakes in general and on the viper in particular. In its attack the hedgehog tries to avoid being bitten, but when, as often happens, it fails to evade a bite the inoculation of the viper’s venom appears to be well borne. This observation has been confirmed experimentally. Phisalix and Bertrand[513] have shown that the resistance of the hedgehog to the viper’s venom is about forty times as great as that of the guinea-pig, that is to say the hedgehog, though far from possessing an absolute immunity, nevertheless exhibits a much greater resistance than do most animals. Lewin[514] convinced himself of this fact as regards adult hedgehogs, though young animals, according to him, are much more susceptible. Thus, he has seen a young hedgehog that had been bitten by a viper die after nine days’ illness. This observation speaks in favour of the conclusion that the immunity of the hedgehog might be naturally acquired rather than a really natural immunity. The hedgehog, hunting all kinds of small animals, might often be bitten by vipers and in this way acquire its immunity against the venom. Under these conditions we can readily conceive that the blood of this “insectivoran” might be placed in a position to develop a specific antitoxic property.

When Lewin tried to satisfy himself of the existence of this property by direct experiment he could only show that the blood of the hedgehog was powerless to prevent the lethal effect of the viper’s venom on small animals. But here, as in his researches on cantharidine, he did not take into account the inherent toxicity of the blood of the hedgehog. Phisalix and Bertrand[515], who have also studied this question, have obtained results at variance with those of Lewin. They demonstrated first of all that the blood of normal hedgehogs was capable of intoxicating and even of killing laboratory animals such as the guinea-pig. It is quite natural, therefore, that the mixture of this fluid with viper’s venom could not be tolerated. It was, however, sufficient to heat the blood of the hedgehog to 58° C. for it to become not only innocuous of itself, but even for it to exhibit an antitoxic action against snake venom. Thus, guinea-pigs which had received 8 c.c. of heated hedgehog’s serum into the peritoneal cavity, were at once in a condition to resist double the lethal dose of viper’s venom. Phisalix and Bertrand conclude, therefore, that “the natural immunity of the hedgehog against the viper’s venom is due to the presence in its blood of an immunising substance.” The same observers[516] satisfied themselves that horse’s serum and even that of the guinea-pig exercise an undoubted antivenomous action; yet these animals are anything but insusceptible to snake venom. Moreover, the necessity to heat the blood to 58° C., as a preliminary measure, deprives this conclusion of the degree of certainty one would like to have in such a matter. On the other hand, the greater susceptibility of young hedgehogs prevents us from putting the immunity of the adult in the category of natural immunity properly so called.

[Sidenote: [356]]

Analogous considerations apply in the case of the mongoose (_Herpestes ichneumon_), carefully studied by Calmette[517], according to whose researches the Antilles mongoose is not very susceptible to snake venom; it readily withstands doses very large relatively to its size, but its immunity is not absolute. It owes much of its mastery in its fights with venomous snakes to its extraordinary agility. The blood of the mongoose, mixed with venom, exhibits an undoubted antitoxic power, though this is not sufficient to prevent the death of susceptible animals. We have no data to enable us to explain the origin of this antitoxic property, but it is probable that here again we have an example of relative immunity, acquired during life. Calmette points out, however, that his ichneumons came from Guadeloupe, where no venomous snakes are found. We may, of course, suppose that the feebly antitoxic power of the blood of these mammals might be due to other snakes or to species of animals whose blood possesses a certain venomous property[518].

[Sidenote: [357]]

We have far more exact data on the natural immunity of certain mammals against toxins of microbial origin. The example most thoroughly studied, one which has become, one might say, classic, is that of the rat against diphtheria toxin. Since the discovery of this toxin, the first well-studied bacterial poison, a discovery made by Roux in collaboration with Yersin, it has been recognised that mice and rats tolerate large quantities of diphtheria cultures or of their filtered products. A rat resists a dose of the diphtheria poison capable of killing several rabbits. To explain this great natural immunity it was suggested that the antitoxic property of the body fluids could be called in. It was supposed that the rat’s blood was, by its very nature, endowed with the power of neutralising the toxin of diphtheria. But, as in the tetanus of fowls, it was not long before facts rendered this hypothesis untenable. Kuprianow[519] studied this question under the direction of Loeffler and gave an account of the results of his experiments, which proved that the blood of the sewer rat, which is very refractory against diphtheria, contains no substance that will neutralise the morbific action of diphtheria toxin on susceptible animals, especially the guinea-pig.

It was necessary to seek some other explanation, and the idea that the immunity of the rat depends on the insusceptibility of its living cells to the diphtheria poison was seized upon. The experiments carried out by Roux and Borrel[520] demonstrated the incorrectness of this hypothesis. The immunity of rats to subcutaneous or intraperitoneal injection of diphtheria toxin is very marked. But a very small dose (0·1 c.c.) of this poison, introduced directly into the cerebral substance of the rat, produces a complete paralysis, which lasts for several days, and ends in the death of the animal. Roux and Borrel conclude from this “that the brain of the rat is specially sensitive to the action of the diphtheria poison, and that as this animal does not die as the result of the injection of large quantities of toxin into the subcutaneous tissue, it is because the toxin does not reach the brain.” These authors have pointed out analogous facts in connection with other examples of natural immunity. The rabbit, which withstands a hypodermic injection of 30 centigrammes of chlorhydrate of morphia, is killed by 1 milligramme only of this salt, introduced directly into the brain. Here, again, neither the cellular insusceptibility nor the antitoxic property of the blood (no “antialkaloidal” power could ever be demonstrated) can explain the immunity, which appears to be due rather to the factor which arrests the poison on its way to the nerve centres.

[Sidenote: [358]]

In spite of the insufficiency of our knowledge as regards natural immunity against soluble poisons we are quite justified in affirming that this category of phenomena comes mainly into the domain of the cells. The body fluids of animals which exhibit this immunity have been found to be antitoxic in a few species only (scorpion, snake, hedgehog, mongoose). And for the majority of these it is possible to invoke special causes, such as the internal secretion of snake and scorpion venoms by the glands which manufacture them, or the acquisition of an antitoxic power during life resulting from wounds or from the absorption of venomous food. The theory of the insusceptibility of the cells of animals naturally refractory to toxins must also be rejected; it is incompatible with well-established facts. Nothing remains, then, but to assume that the formed elements are the principal factors in this natural immunity, and that they interpose to prevent the passage of the poisons towards the very susceptible nerve cells.