INFECTION AND IMMUNITY. 563 



that has occurred in the tissue cells, as a result of the 

 primary infection or intoxication, which serves to pro- 

 tect 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 diph- 

 theria 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 progressive in- 

 quiry. The triumph resulting from the practical applica- 

 tion of this principle to the prevention and cure of diph- 

 theria 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, tuber- 

 culosis, 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 researches that have served to indi- 

 cate not only the manifold nature of this complex prob- 

 lem 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, 

 *'. e., that it depends upon the specific peculiarities of 

 the invading bacteria. In some instances it is mani- 

 fested as an effort on the part of the tissues to neutralize 

 bacterial poisons, the bacteria themselves remaining un- 

 affected ; in others as an actual destruction, disintegra- 

 tion or digestion of the invading bacteria together with 



