212 STUDIES IN IMMUNITY. 



Our theory makes two assumptions: First, that cytolytic phe- 

 nomena in vitro, produced by adding the sensitive cell to the two 

 substances of active serum, are quite similar to those which occur in 

 vivo with thi' same substances. This may indeed be experiment- 

 ally demonstrated provided that relatively similar amounts of each 

 substance are present in both cases. For example, we find that the 

 smallest amount of cholera sensitizer that will cause a given amount 

 of vibrios to be transformed by alexin in vitro is similar to the 

 amount necessary to destroy the same number of vibrios in a nor- 

 mal peritoneal cavity. This fact has been already mentioned. 



Second, that a normal animal injected with an immune serum 

 acts simply as a passive container for the sensitizer. The animal 

 receives the substance, but does not modify it, and it becomes diluted 

 in the fluids of the body, particularly in the blood. The serum 

 from this injected animal should act, then, like a dilution of the sen- 

 sitizer in normal serum. This we believe we have proved experi- 

 mentally.* This conclusion holds as well for the agglutinating 

 property.! .,. 



Pfeiffer' s theory, (1896). — The preceding theory has not been 

 the only one to awaken interest in the study of bactericidal sera. 

 A year later Pfeiffer offered an explanation of the granular trans- 

 formation of vibrios that differs from our own in several important 

 particulars. Pfeiffer, who has carefully studied the bacteriolytic 

 changes in vibrios in the peritoneal cavity, draws a sharp dis- 

 tinction between such phenomena and those that take place in 

 vitro; the latter, according to this investigator, are much less 

 energetic. As Pfeiffer understands it, vaccinated animals have a 

 specific antibody that has two forms : a stable and inactive form 

 that may be kept for a long time, and an active bactericidal form 

 which is less stable. The antibody can easily change from one 

 form to another. The change from inactive to active form is 

 brought about by means of a ferment-like substance of cellular 

 origin; this change takes place easily in the animal body, so that 

 the antibody can attack the vibrios in its most destructive form. 

 Serum, on the contrary, contains the antibody in its stable, inac- 



* See pp. 167 to 171. 



t See, "The mode of action of preventive sera," p. 81. 



