THE DEFENSES OF THE BODY 273 



an animal that has recovered from a specific infection is 

 explainable by a "reactive change" that has occurred in the 

 tissue cells, as a result of the primary infection or intoxica- 

 tion, which serves to protect the animal from subsequent 

 attacks of a similar character. 



The demonstration that the serum of an artificially 

 immunized animal can not only confer immunity upon 

 another animal but, in the case of tetanus and diphtheria 

 in particular, actually cure it after the disease is in progress, 

 is one of the most important steps that has been made in 

 this entire field of inquiry. The triumph resulting from the 

 practical application of this principle to the prevention 

 and cure of diphtheria in man fairly marks an epoch in 

 modern medicine. Though the results attendant upon the 

 application of that principle to the prevention and cure of 

 a number of other diseases — Asiatic cholera, typhoid fever, 

 lobar pneumonia, infection by the pyogenic cocci, rabies, 

 tuberculosis, plague, syphilis, and snake bites — ^have met 

 with comparatively indifferent success, still the knowledge 

 gained through these efforts has been of inestimable value 

 in stimulating researaches that have served to indicate not 

 only the manifold nature of this complex problem but have 

 led to discussions through which some of its most obscure 

 phases have been illuminated. 



Briefly stated, the outcome favors the conclusions that 

 the mechanism of immunity varies in different diseases, i. e., 

 that it depends upon the specific peculiarities of the invading 

 bacteria. In some instances it is manifested as an effort on 

 the part of the tissues to neutralize bacterial poisons, the 

 bacteria themselves remaining unaffected; in others as an 

 actual destruction, disintegration or digestion of the invad- 

 ing bacteria together with the neutralization of such intra- 

 18 



