94 STUDIES IN IMMUNITY. 



ner, is the question of the specificity of sera and the value of this 

 specificity as a means of certain diagnosis of bacterial species. The 

 diagnostic method founded on the instance of specificity offered 

 by Pfeiffer is well known. The serum of an animal vaccinated 

 against the cholera vibrio, for example, when injected together with 

 a culture of the true vibrio into the peritoneal cavity of a normal 

 guinea-pig causes loss of motility and then rapid transformation of 

 the vibrios into roundish granules. We are safe in concluding that 

 all vibrios which give this reaction in presence of an anticholera 

 serum are true cholera vibrios. According to Pfeiffer, all the true 

 cholera vibrios do act in the same way, which fact gives us a means 

 of distinguishing Koch's vibrio from similar vibrios and from other 

 species of bacteria. 



This method, however, has not appeared to all to be absolutely 

 certain. When we say, indeed, that sera are specific and that one 

 may utilize this specificity as a means of diagnosis two distinctly 

 independent facts are implied, each one of which demands a separate 

 demonstration. It means, in the first place, that the animal body 

 during vaccination acquires certain new properties which are in a 

 direct and intimate relation to the kind of bacteria that have been 

 used in immunization. It would seem that this fact has already 

 been justified by experiment ; we have found, for example, that the 

 serum of a guinea-pig vaccinated against cholera is specifically 

 bactericidal for the true cholera vibrios and has little or no effect 

 on vibrios that are knowm to differ from the cholera vibrios, for 

 example, the vibrio Metchnikovi.* But it is at the same time 

 asserted that bacteria which have once reacted in a positive or 

 negative manner to a given preventive serum will thereafter always 

 act in the same way whatever may be their subsequent condition 

 of existence. This statement implies that there are no transitional 

 forms between organisms which react positively on the one hand and 

 those which act negatively on the other; such a lack of transitional 

 forms, indeed, is necessary to render the method precise. This 

 conception, which tends to establish distinct, insuperable limitations 



* These facts were demonstrated first by Pfeiffer. We had previously noted 

 a similar phenomenon, namely, that the serum of animals vaccinated against the 

 cholera vibrio when injected into a normal guinea-pig endows the serum of this 

 animal with intense bactericidal power against the cholera vibrio and against 

 this vibrio only. 



