CHAPTER XX 



FACTS AND PROBLEMS OF IMMUNITY IN THEIR BEARING UPON 

 THE TREATMENT OF INFECTIOUS DISEASES 



While the various facts and theories of immunity and infection have 

 been given in the preceding sections, no systematic attempts have been 

 made to correlate the facts presented, or to determine their bearing on 

 the most vital problem of all — the treatment of infectious diseases. 



To vmderstand more fully this point of view, it is necessary briefly 

 to recall certain of the facts which are known about the physiology, 

 metabolism, and composition of the bacteria, and of their ability to 

 neutralize directly or to respond adaptively to the agents directed against 

 them by the invaded animal. Some of these facts are so well understood 

 that passing mention here is sufficient: such, for instance, is the fact 

 that certain microorganisms, especially the bacilli of diphtheria and tet- 

 anus, secrete soluble poisons both during artificial cultivation and dur- 

 ing their life in the animal body, which poisons are eminently toxic. 

 These poisons are true secretions and are largely independent of the 

 composition of the surrounding medium so long as this favors the physi- 

 ologic activities and growth of the germs. Such germs, then, once having 

 gained even an insecure foothold in the animal body, by no matter what 

 favoring circumstances, are possessed of a powerful weapon of offense 

 against the sensitive physiologic bases of the host and, possibly, of de- 

 fense against its more immediate and mobile means of combating the 

 germs themselves. In the case, however, of most other pathogenic 

 bacteria, the secretion, at least in artificial media, of such highly soluble 

 and potent poisons has not been demonstrated satisfactorily, although 

 certain investigations point fairly conclusively to the production of 

 some minor bodies which have been shown to act deleteriously on the red 

 blood cells and on the leucocytes — the hemolytic, leucocidic, and leu- 

 colytic substances which are looked on as probably true soluble toxins, 

 like the toxins of diphtheria and tetanus, which give rise in the animal 

 body to the production of true antitoxins: i.e., are neutralized by 

 their antisera, unit for unit, according to the law of multiples. 



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