# Popular Science Monthly/Volume 38/January 1891/Predisposition, Immunity, and Disease

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* PREDISPOSITION, IMMUNITY, AND DISEASE.
By W. BERNHARDT.
IT is a generally recognized fact that whole classes and families of animals, as well as single individuals, frequently are liable to succumb to some influence apparently obnoxious to health, while others, although exposed to the same danger, prove exempt from such injury. This experience concerns the action of vegetable and animal poisons, as well as the attacks of the various diseases to which flesh is heir. Destitute of a satisfactory interpretation of these divergences, we have recourse to the expression "predisposition" for explaining the inability of offering ​resistance to the foe—a word which, does not actually explain the matter, but furnishes a convenient term. Germs of disease are to be found everywhere, but only predisposition permits its development. Immunity, on the contrary, is the condition of the system which prevents an outbreak. The fundamental cause of this condition is as little known as the cause of predisposition; only in a few cases have we been successful in tracing it back to certain chemical and physiological processes occurring in the body.
The action of carbon monoxide on different animals affords a suitable instance of what is called immunity, and illustrates the kind of circumstances on which it may sometimes depend. Carbon monoxide is an air-like compound, which is contained to a large amount in the illuminating gas produced by the decomposition of steam by red-hot coals, and to the presence of which the poisonous qualities of this gas are chiefly due. A mixture of one part of carbon monoxide and ninety-nine parts of common air, when breathed, will in a short time kill any of the warm-blooded vertebrates. Cold-blooded vertebrates, such as frogs, can for a considerable length of time stand the exposure to such an atmosphere; arthropoda or insects are not in the least affected by it—they possess immunity from it. Searching for the cause of these differences of effect, we find it to be the tendency of hæmoglobin, the albuminous matter constituting the red corpuscles of the blood, to combine with carbon monoxide. In the process of respiration in warm-blooded animals hæmoglobin takes up oxygen, which thereafter, as a necessary agent in the exchange of matter, is delivered to the different organs of the body. Carbon monoxide prevents the absorption of oxygen, being absorbed in its place; but, unfit as it is to replace oxygen in its vital functions, it causes serious derangements, which end in suffocation. In cold-blooded vertebrates respiration is of more subordinate importance; although, as well as in warm-blooded animals, it consists in absorption of oxygen by hæmoglobin, the need for oxygen is much lower; a frog can live for a considerable time without the accession of air. Hence the effect of carbon monoxide is a much slower one. The blood of insects contains no hæmoglobin; carbon monoxide is not absorbed by it, and is not a poison to them, provided that a sufficient amount of oxygen is always present. Carbon monoxide, consequently, acts as a strong poison upon warm-blooded animals; its effect is weaker in cold-blooded vertebrates; and insects are proof against its effects.
In a few instances only has the cause of immunity become as well disclosed as in the one mentioned. Neither differences of organization in animals nor in the constitution of the poisonous substance generally afford any clew for interpreting an exceptional want of effect. Unaccountable is the immunity of rabbits against ​belladonna leaves (Atropa belladonna, deadly nightshade). You may feed them with belladonna for weeks without observing the least toxic symptoms. The meat of such animals, however, proves poisonous to any one who eats it, producing the same symptoms as the plant. Pigeons and various other herbivora are also to some degree safe from the effects of this poison, while in warm-blooded carnivora it causes paralysis and asphyxia. In frogs the effect is a different one, consisting of spasms. The meat of goats which had fed on hemlock has sometimes occasioned poisonous effects. Chickens are nearly hardy against nux vomica and the extremely dangerous alkaloid, strychnine, contained in it, while in the smallest amount it is a fatal poison to rodents. More remarkable yet in this respect is the immunity of Choloepus Hoffmanni, a kind of sloth, living on the island of Ceylon, which, when given ten grains of strychnine, was not much affected. Pigeons are possessed of high immunity from morphine, the chief alkaloid of opium, as well as from belladonna. Eight grains were required to kill a pigeon, not much less than the mortal dose for a man. Cats are extremely sensitive to foxglove (Digitalis purpurea), which on the contrary may be given to rabbits and various birds in pretty large doses. Many kinds of fish may be killed by just a trace of Cocculus indicus, although their meat is not made injurious by it. Laughing-gas, or nitrogen monoxide, a means used to relieve pain in light surgical operations, affects man more than any other creature; when breathed in a mixture of four parts of laughing-gas and one part of oxygen it produces a pleasant kind of intoxication together with diminished sensibility, though in animals no such effect has been observed.
The immunity of certain animals against the bite of venomous serpents is remarkable. Numerous observations have been recorded proving the polecat, hedgehog, and buzzard to be proof against the bite of the viper; it is mortal for most other animals of the same size and nearly related to them.
Immunity, however, is not limited to the relations of animals to poisons of vegetable or animal origin, but is manifested as well in conditions and processes in the healthy animal organism and in its susceptibility to diseases. The resistance offered by the living stomach of an animal to the dissolving effect of the juice secreted by the stomach itself has to be explained by immunity. A watery solution of pepsin—the digestive principle of the stomach—acidulated by muriatic acid, and thus, as to composition, corresponding to the digesting juice of living animals, upon addition of pieces of the stomach of any mammal, dissolves them, forming a perfect solution. The stomach of the living healthy animal, on the contrary, does not undergo the least change by the secreted juice; it is proof against the digesting effect of its ​own secretion, as well as to a certain degree against various sickening external influences.
Prominent naturalists are at present occupied in inquiring for a reasonable way of interpreting the causes of sickness and the conditions of immunity from it, or the resistance offered by a sound organism. Sickness, as well as health, according to one of the prevailing theories, depends upon chemical causes, viz., on the presence and predominance of various complex substances generated in the juices and tissues of the body by unknown processes, in which bacteria may sometimes play an important part. According to another theory, the living animal cells are engaged in a continual struggle against intruding micro-organisms. Animal cells are considered as individuals similar in character to the order of Amœbæ, which are unicellular organisms of the class of Protozoa. Metschnikoff found that certain cells of the animal body are endowed with the faculty of swallowing and digesting intruding bacteria of every kind, harmless ones as well as pathogenic ones, or such as produce disease. Not all elementary organs of the body are equally qualified for this purpose, the function being intrusted to certain cells of the tissues and blood, which Metschnikoff calls phagocytes. Health as well as disease depends upon which party is victorious in the struggle. Health is insured as long as the cells are capable of overpowering the intruding bacteria; an animal in such a condition is secure against disease. Experiments performed by Metschnikoff have given evidence that the bacilli of splenic fever are easily devoured and digested by phagocytes. On the other hand, several observers of late have maintained that the liquid part of blood, the plasma, and even common albumen, possess the faculty of killing bacteria. This, however, appears improbable, and a final decision of the question has still to be expected in future.
Susceptibility to diseases is as variable as sensitiveness to vegetable and animal poisons. Judging from the current opinion that putrefying animal matter is the principal bearer and transporter of infectious germs, we are forced to ascribe a high degree of immunity to certain animals which, like swine, ducks, chickens, and rats, are accustomed to select their food from places where such matter is accumulated. Predisposition for splenic fever is stronger among herbivora than among carnivora; birds of prey seem to be quite free from it. Experiments on sheep, performed by Pasteur, the results of which were confirmed by application on a large scale, gave evidence that immunity against splenic fever may be acquired by systematic inoculation of the attenuated virus very much as small-pox is prevented by vaccination.
Various herbivora, chiefly horses, sheep, and goats, are exposed to a disease called "glanders," which ends by death in most cases. ​White mice are safe against it. This circumstance of late occasioned R. Koch to ascertain, by experiments, whether predisposition to glanders might not be artificially induced by changing the composition of the animal juices. The change consisted in the formation of sugar in the blood of the mice, which received as food phloridzin, a crystalline compound, naturally preformed in the roots of fruit trees and easily splitting up into sugar and some other products. It undergoes a similar change when brought into the circulation of the blood. The result of these experiments was, that white mice lose their immunity and become susceptible to glanders when phloridzin is given to them; infection by this disease invariably took place when the mice were inoculated to the virus, and thus the proof was furnished that by changing the chemical conditions of an animal its immunity from infectious disease may be neutralized. This indicates that immunity in the present case, as in the action of carbon monoxide, depends upon the composition of the blood, predisposition being established when the composition is changed.
These facts indicate that, as to susceptibility to and immunity from the effect of poisonous and virulent matter, the composition of blood is of the highest signification, and that the changes caused chiefly relate to its condition. They coincide with the experience that the action of poisons throughout is quickest and most energetic when they are injected into the blood; moreover, there seem to be many substances existing which induce infection only when present in the circulation of the blood, but not when brought into the digestive channel. Apparently harmless lesions can turn out disastrously, when even the smallest trace of a virus happens to reach the wound.