THEORIES OF IMMUNITY. 



Both active and passive immunity can be 

 secured in such diseases, but the latter only to a 

 very slight degree. 



The active immunity secured is obtained by 

 the injection of non-fatal or attenuated cultures 

 of the bacteria — or by the use of virulent cul- 

 tures that have been killed by heat (as in the 

 procedure of Wright and Semple against t} 7 phoid 

 fever). 



The passive immunity is of the slightest de- 

 gree, and the curious fact is developed that 

 in both typhoid and cholera the serum of 

 animals treated with dead cultures of the bacteria 

 of these diseases is bacteriolytic to the living and 

 virulent bacilli, but not antitoxic to the toxine. 



Bordet's publication of his results in the study 

 of hemolysis, and the light they threw upon 

 Pfeiffer's phenomenon, were the first steps tow- 

 ards the explanation of the phenomena seen in 

 the second group of bacterial infectious processes. 

 Pfeiffer's phenomenon is one of a purely bac- 

 teriolytic nature (of course bactericidal also, 

 although a bactericidal serum need not be bac- 

 teriolytic), and one of the ways in which it can 



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