Facts bearing on acquired immunity 215 



separately, a few cholera vibrios are added, they are quickly trans- 

 formed into granules. This transformation, obtained in vitro, is 

 remarkably like that produced in the peritoneal cavity of the vac- 

 cinated animal. 



Jules Bordet 1 , working in my laboratory, made a very com- 

 plete investigation of Pfeiffer's phenomenon outside the animal body 

 and found that, in my experiment, the peritoneal lymph can be 

 replaced by the blood serum of the vaccinated guinea-pig without 

 thereby in the least hindering the granular transformation. After 

 making a specially thorough study of the question he has come 

 to the conclusion that Pfeiffer's phenomenon is the result of the 

 action of two substances. One of these is found in the blood serum 

 and in the peritoneal fluid of guinea-pigs vaccinated against cholera, 

 heated to 55 56 C. or deprived by some other means of their indi- 

 vidual power of transforming vibrios into granules. This substance 

 resists this temperature and only loses its activity on being heated 

 to 68 70 C. The second of the two substances, that found in the 

 peritoneal lymph or in the blood serum of the normal guinea-pig, 

 is, on the other hand, destroyed at 55 56 C. and is nothing but the 

 ordinary cytase (or alexine) present in the fluids of normal animals. 



The facts we have described with regard to Pfeiffer's phenomenon 

 in the body fluids of immunised animals must, then, be interpreted 

 as follows. The fresh peritoneal exudation or blood serum of these 

 animals readily produces the granular transformation, because in 

 these fluids both the two necessary substances are found. But as 

 microcytase is a very unstable substance which, under the influence [2-27] 

 of time or heating to 55 56 C., is destroyed, the fluids of im- 

 munised animals are very readily deprived of it The blood serum 

 then, after being some time outside the body, becomes incapable of 

 transforming vibrios into granules ; but when to it is added a small 

 quantity of the cytase, found in the blood serum or in the peritoneal 

 lymph of the normal guinea-pig, the transformation takes place with 

 great rapidity. To the serum of the immunised animal, which has 

 become inactive, is restored its property of setting up Pfeiffer's 

 phenomenon. This interpretation, formulated by Bordet, correspond- 

 to the whole of the known data and is now generally accepted. 



As the fluids of immunised animals, that have become incapable 

 of transforming vibrios into granules, still retain their power of 

 rendering these organisms motionless and of uniting them into 

 1 Ann. de I'Inst. Pasteur, Paris, 1895, t. ix, p. 4ii2. 



