78 STUDIES IN IMMUNITY. 



cholera vibrio by Pfeiffer. The analogy is so complete that it 

 seems justifiable to assert that the substance that gives rise to the 

 morphological change in the colon bacillus, the typhoid bacillus, 

 and the phosphorescent vibrios is of leucocytic origin as well as in 

 the case of the cholera vibrio. Indeed, when B. typhosus or 

 B. coli is mixed with leucocytes from a normal guinea-pig the 

 transformation occurs only inside cells. Oval granulations are 

 formed there similar to the cholera granulations. It is evident, 

 then, that Pfeiffer's phenomenon, whether with the colon bacillus or 

 the typhoid bacillus or Koch's vibrio, follows the same rule. The 

 colon bacillus and the typhoid bacillus give further examples of 

 the fact that phagocytes may become so active in well-immunized 

 animals as to bring about a diffusion of their substances into the 

 surrounding fluid. 



We have already seen that the bactericidal substance that affects 

 vibrios is not in itself specific. It is the preventive substance that, 

 while adding to its activity, endows it with specificity. We have 

 shown, accordingly, that bactericidal sera from animals immunized 

 against various organisms are to be distinguished only by the 

 specificity of their preventive substance. The bactericidal sub- 

 stance, properly speaking, in immunized animals is, with the possible 

 exception of certain quantitative changes, quite the same as in 

 normal animals. And since Pfeiffer's phenomenon occurs with bac- 

 teria other than the cholera vibrio and under identical conditions, 

 it seems evident that the bactericidal substance that phagocytes use in 

 their struggle against bacteria is the same, whatever may be the organism 

 attacked. In an animal immunized against a given infection the 

 bactericidal substance is directed against the specific organism on 

 account of the presence of the preventive substance, the specificity of 

 which depends on the organism used for immunization. It is due to 

 the presence of this peculiar preventive substance that the animal 

 directs its destructive power against a given organism. What is 

 the nature of this active and distinctive body? By what mechanisn 

 have the body cells been enabled to create this specific substance 

 that is so valuable to the animal? This is one of the most important 

 problems of cellular biology, and, at the same time, at present the 

 greatest enigma with which immunity confronts us. 



