IMMUNITY AND VACCINE THERAPY 481 



further existence and died out leaving the body unsuited for 

 future occupation. The theory could not apply to passive 

 immunity produced by the injection of antitoxin. 



2. The retention theory. This was proposed by Wer- 

 nich and Chauveau. This theory is based on the supposition 

 that bacteria elaborate some metabolic product that inhibits 

 their further development and the future invasion of the 

 tissues by the same species. This theory is illustrated by the 

 facts shown in the cultivation of bacteria in artificial media. 

 The bacteria often die apparently from the accumulation of 

 metabolic products long before the nutriment is exhausted. 



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 be- 

 cause 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 

 5.5 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 must exist an active central body 

 and a number of other chemical groups or side chains. These 



