IMMUNITY 201 



animal no longer killed fresh animals : but this was due to the 

 fact that they were saturated with the immune serum which 

 contained immune-bodies : deprived of these by thorough 

 washing, they regained their original virulence. 



It has further been said that the microbes act through their 

 toxins and that the body-fluids of an immunized animal begin 

 by neutralizing this toxicity, after which the bacteria fall easy 

 victims to the phagocytes. But if this were the case why should 

 there be such profound differences between the immunity 

 towards the microbes and that towards their toxins? Why 

 should there be in animals immunized against the bacillus 

 pyocyaneus or the cholera vibrio a complete resistance to infec- 

 tion with these microbes along with a susceptibility to the 

 toxins equal to that of a fresh animal ? 



In all these objections to phagocytic immunity it is always 

 the idea of a direct primitive action of the body-fluids which 

 appears, the idea of the humoral theory. Since it is admitted 

 that the immune-bodies circulate in the plasma whereas 

 according to Metchnikoff the complement remains in the 

 phagocytes, since the phagocytic theory maintains that no 

 excretion of complement occurs without phagolysis, it ought 

 certainly to have been on this point that the humoral theory 

 should have made its attacks. This is the central point on 

 which turns the whole question : if without phagolysis there is no 

 extracellular destruction of microbes in an immunised animal, 

 when destruction takes place outside the phagocytes it means 

 there has been an abnormal lesion, a phenomenon unlikely to 

 occur spontaneously in nature and probably only an experi- 

 mental accident ; it was the celebrated experiment of Pfeiffer 

 which threw the question into prominence. 



Pfeiffer's Phenomenon and the Humoral Theory 



It was the following experiment of Behring and Nissen which, 

 after the primary investigations of Fliigge, Nuttall and Buchner, 

 seemed best to explain immunity by the bactericidal power of 

 the body-fluids ; the serum of guinea-pigs well vaccinated against 



