50 STUDIES IN IMMUNITY. 



As is seen by this table the serum of the guinea-pig before injec- 

 tion of immune serum was not strongly bactericidal either for the 

 vibrio Metchnikovi or the cholera vibrio. After injecting the pre- 

 ventive serum, the serum of the guinea-pig became distinctly 

 bactericidal for the vibrio Metchnikovi, but not at all so for the 

 cholera vibrio. 



The preventive value of a given serum against the vibrio Metch- 

 nikovi and against the cholera organism may be compared. The 

 animal treated with serum acquired a means of defense solely 

 against the vibrio Metchnikovi. It is not proper then to call a 

 serum preventive against such and such a vibrio unless inoculation 

 of this serum into an animal endows the latter with a marked bac- 

 tericidal property for the organism in question. The experiment 

 offered gives an example of the specificity of the preventive power. 

 It by no means proves that there is no cholera vibrio or never will 

 be a cholera vibrio similar enough to the vibrio Metchnikovi, so that 

 an infection by such an organism might not be prevented by a 

 serum active against the vibrio Metchnikovi. It indicates simply 

 that the preventive substance formed by the animal body bears a 

 distinct relation to the bacterium which has formerly attacked the 

 animal and against which it has become immunized. 



We know that the bactericidal activity of immune serum is di- 

 rected only against that species of vibrio used for immunization. To 

 what is this specificity in bactericidal power due? We have already 

 seen that the bactericidal substance in the serum of immunized 

 animals does not differ from the weak non-specific substance in 

 normal animals. The specificity and strength of the bactericidal 

 substance in the serum of immunized animals must be due to the 

 preventive substance. The specificity of the bactericidal substance 

 indicates the specificity of the preventive substance, and the pre- 

 ventive property is specific to the same degree that the bactericidal 

 power is. 



VIII. SOME CHARACTERISTICS OF THE PREVENTIVE SUBSTANCE. 



The preventive substances present in the sera of animals immun- 

 ized against different organisms are not identical. The fact that an 

 infection produced by one vibrio cannot be prevented by the serum 

 of an animal immunized against another vibrio is an evidence of 



