THEORIES 551 



the cultivation of bacteria in artificial media. The bacteria 

 often die apparently from the accumulation of metabolic pro- 

 ducts long before the nutriment is exhausted. After these 

 cultures are filtered and the bacteria present are removed, 

 other bacteria of the same species will not grow in it. This 

 theory, however, does not explain immunity against lysins, 

 serpent's venom and other poisons. 



3. The phagocytosis theory. Metchnikoff has supposed 

 that acquired immunity is brought about because of the action 

 of the phagocytes upon the invading organisms. He has 

 shown that in cases of infection with the Vibrio Metchnikovi, 

 the phagocytes of unprotected animals do not take up the 

 bacteria, but that in vaccinated animals they do. It appears 

 from all the work that has been done on this subject that the 

 phagocytes are active in proportion to the degree of immunity 

 possessed by the individual. It has not been demonstrated, 

 however, whether they are active because the animal is 

 immune and the bacteria harmless to it, or whether the animal 

 is immune because the phagocytes are destructive to the 

 bacteria. 



4. The humoral theory. This theory is based on the 

 observations of Buchner, Nuttall and others that blood serum 

 has the power of destroying a certain number of bacteria when 

 introduced into it. Nuttall showed in addition to this that the 

 bacteriolytic power ceased if the blood was heated to 55° C. 

 It is found, however, that the bacteriolytic serums occur only 

 in cases where there is a high degree of forced immunity, 

 their activity being in proportion to the degree of immunity 

 obtained. An explanation for the action of these serums upon 

 bacteria is given by Ehrlich in his lateral-chain theory. 



5. Ehrlich' s side-chain theory. According to Ehrlich, in 

 every living cell there mu.st exist an active central body and a 

 number of other chemical groups or side chains. These groups 

 have the greater variety of function, especially those of nutri- 

 tion and assimilation. This theory teaches that immunity 

 depends upon the presence or absence of certain substances 



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