n6 THE POPULAR SCIENCE MONTHLY 



Further study showed that the various bacterial toxins produce not 

 only a fatal intoxication, but that each has its distinctive effect, as 

 shown by symptoms or anatomical lesion, when injected into animals, 

 thus demonstrating that the poison of each bacterium possessed a spe- 

 cific action. This led not only to a better understanding of the pathol- 

 ogy of such diseases as diphtheria and tetanus, but eventually, and of 

 far greater .importance, to the discovery of curative and prophylactic 

 sera, or as they are generally known, antitoxic sera. The first step in 

 this direction was taken when Behring and Kitasato (1890) showed 

 that animals could be immunized against weakened diphtheria toxin 

 and that the serum of such animals is capable of protecting other 

 animals against its intoxication, and, moreover, demonstrated that such 

 a serum can be used to cure the toxic symptoms produced by the 

 diphtheria bacillus. This curative power, furthermore, was found to 

 be due not to an action on the bacteria, but to a neutralization of the 

 toxin which the bacteria produced ; also the serum was strictly specific, 

 that is, the serum of an animal immunized against diphtheria toxin 

 protects only against diphtheria; that prepared by the use of tetanus 

 bacilli, only against tetanus. This led directly to the production by 

 Behring and Knorr of diphtheria antitoxin for therapeutic purposes 

 (1894) on a large scale and to a general awakening as to the possibili- 

 ties of serum therapy. The great benefits of diphtheria antitoxin as a 

 curative and prophylactic serum are known to all ; since its general use, 

 in 1896, a reduction of the death rate in diphtheria from 45 per cent, 

 to 10 per cent, marks this therapeutic measure as one of the most bril- 

 liant discoveries of medicine and of the brilliant century in which this 

 discovery occurred. 



The success with diphtheria antitoxin aroused the hope that a gen- 

 eral principle — that of the formation of antibodies for the toxins of all 

 bacteria — had been established on the basis of which it would be possible 

 to develop curative sera for all infections. This expectation — on ac- 

 count of the simple fact that most bacteria do not produce soluble 

 poisons — has not been fulfilled ; but the impetus which the principle of 

 serumtherapy gave to investigation has led to activity of great and 

 permanent value, and to the development of a new science, immunology 

 or serology, as it is variously called, which attempts to establish laws 

 for the conditions which determine natural resistance to infectious dis- 

 eases and the factors which increase or diminish this resistance. I 

 approach this subject with hesitation, for the many difficulties it offers 

 can not readily be overcome in a short presentation such as this must be. 

 A few brief statements, stripped of the less familiar terms may, how- 

 ever, serve to elucidate the main lines of investigation. 



All immunological studies are based on the known fact of the rein- 

 forcement of natural resistance to disease, as illustrated by serum 



