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Immunity

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IMMUNITY.

Natural Immunity.—This is the term used to imply that a person has a degree of insus ceptibility to any disease. We all know people who are very susceptible to any disease they come in contact with, and in sonic cases the susceptibility is a characteristic of a family. Sometimes it is to one kind of disease, infectious disease in particular, sometimes it is a general susceptibility. In the same way there are per sons who have a more marked power than ordi nary of resisting disease, whom epidemics leave untouched. These people are said to be im mune, or to have immunity, and, just like susceptibility, it may be an individual or a family characteristic, and may apply to certain types of disease or to disease in general. There are diseases, also, which attack animals, to which men are not prone, and similarly there are dis eases of Inman beings which do not naturally attack animals; for instance, typhoid fever and cholera do not attack the lower animals. That is to say,..hunuin beings are immune to some diseases to which animals are prone, and vice versa.

Acquired Immunity.—It is a familiar fact that, in the case of most of the infectious dis emes, one attack protects against another, more or less completely; that is to say, by the one attack the person has become immune to the disease; immunity, that is to say, has been acquired. This was always a striking fact in the observa tion of disease, and reference to the paragraphs on small-pox will show how it was made use of in order to protect from the grave form of that disease, multitudes being, at risk of life, de liberately inoculated with the disease, which, introduced in this way, ran a milder and less disfiguring course, rather than incur the risk of taking it in the ordinary way.

But it was not till it became clear that in fectious diseases were due to the introduction into the body of living organisms, and were the expression of the growth and multiplication of these organisms in the body, that the full significance of the fact of acquired immunity became apparent. For, if each infectious dis ease were due to one special organism, if the symptoms of the disease were due to the change going on in the body by its growth, if one attach of the disease protected against a second, and if the organism could be separated and grown pure outside the body, there was an obvious possibility of producing the changes in the body on which immunity depended, with less risk and in shorter time, by the use of the artificially reared organisms. It has already been ex

plained (p. 501) how this idea occurred to Pasteur, and how the possibility was converted by his experimental genius into certainty in the case of anthrax.

Artificial Immunity is the proper term to use when an animal or person has been pro tected in the way indicated in the last para graph from an attack of a particular infection. Up to a comparatively recent time, vaccination against small-pox was the only kind of artificial immunity practised on the human being. In animals, ou the other hand, the possibility of artificially conferring protection has been proved in quite a number of diseases.

It has been pointed out (p. 604) that the phenomena of fever may be due— (I) Either to the growth and multiplication of the organism of the disease in the blood and tissues of the person attacked, or (2) To the poisonous action of the chemical substances produced by the organisms, or (3) Partly to one and partly to the other.

It has been proved, for instance, that in some diseases all the symptoms can be produced by the introduction into the body of the material in which the organism has been grown, but from which all organisms have been removed by filtration. Such a liquid would contain all the soluble poisons produced by the agency of the organism in the fluid in which it was culti vated, but none of the organisms. This is true of diphtheria and tetanus (lock-jaw). In such diseases, therefore, it is not so much the mere growth and multiplication of the organisms in the body that produce the features of the dis ease as the poisons (toxins) which they manu facture, and which are circulated with the blood, poisoning the tissues. In other diseases, of which tuberculosis is an example, the intro duction of the toxins may produce symptoms, but not the characteristic developments, of the disease, the living organism being necessary for that.

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