248 INFECTION AND IMMUNITY. 



vention of the animal body, by mixing perfectly 

 fresh anticholera serum with the vibrios and 

 mounting as a hanging-drop preparation. The 

 slide must be kept at the temperature of the body 

 by means of a warm stage. The reaction, how- 

 ever, is far less vigorous than when it takes place 

 in the peritoneal cavity and the solution of the 

 cells may not be complete. No bacterium is so 

 completely dissolved under these conditions as the 

 vibrio of cholera, although the typhoid bacillus and 

 similar organisms undergo some changes in their 

 form. 



Activa- The experiment of Pfeiffer may also be con- 

 active serum ducted in the abdominal cavity of a non-immune 

 by the g Tis- g u i nea _pjg by injecting anticholera serum in con- 

 junction with the culture (passive antibacterial 

 immunization) . This is the classic Pfeiffer experi- 

 ment. The immune serum should be of such 

 strength and should be given in such quantity that 

 the animal is saved in spite of the ten fatal doses 

 of culture which the typical experiment demands. 

 Experiments brought to light a condition which 

 seemed paradoxic; an old immune serum which 

 had lost its bactericidal power as manifested in 

 vitro, or one in which the alexins had been de- 

 stroyed by a temperature of 60 C., showed its 

 original protective power in the animal experi- 

 ment. Furthermore, when an inactive immune 

 serum was injected into the abdominal cavity, al- 

 lowed to remain for a time and then withdrawn, 

 its bactericidal power for experiments in vitro was 

 found to be re-established. On the basis of these 

 facts, Pfeiffer concluded that the specific sub- 

 stance is present in the immune serum in an inac- 

 tive form, and that it becomes active as a result 



