214 Chapter VIII 



peritoneal cavity of an immunised guinea-pig, the animal recovers 

 from a malaise that is quite transitory and continues to live, whilst 

 normal unvaccinated guinea-pigs die, an enormous quantity of vibrios 

 swarming in the peritoneal exudation. The difference between the two 

 animals is most striking, and we can readily understand that Pfeiffer 

 was so impressed by it that he was led to attribute the acquired 

 immunity of his guinea-pigs solely to the granular transformation 

 set up by a bactericidal substance contained in the fluids of the 

 immunised animals. 



The ease with which we can gain an idea of the change of form 

 in the vibrios under the influence of the fluids of the body, greatly 

 aids the study of the bactericidal substance. Before passing to the 

 question of the part played by this substance in acquired immunity 

 we must consider for a moment the principal properties of this acquired 

 immunity. Very manifest in the peritoneal fluid, the power of causing 

 Pfeiffer's phenomenon is equally evident in the blood serum of im- 

 munised guinea-pigs, as has been demonstrated by Bordet. A drop 

 of this serum, when quite fresh, readily and rapidly transforms a 

 number of vibrios into granules. When the serum is kept for several 

 days or has been heated to 55 C. for an hour, the total disappearance 

 of the substance which produces Pfeiffer's phenomenon is brought 

 about. This at once betrays the presence of microcytase in the 

 fluids of guinea-pigs that have acquired immunity against the cholera 

 vibrio. Yet the blood serum and the peritoneal fluid of these 

 animals, having been deprived of their microcytase by heating 

 to 55 or 56 C.> still retain a remarkable power over the vibrios. 

 These organisms no longer undergo granular transformation, under 

 [226] the influence of the heated body fluids, but they are deprived of all 

 power of motion, agglutinate into clumps and acquire a special 

 susceptibility to the action of cytase. Soon after the discovery of 

 Pfeiflfer's phenomenon, I 1 was able to demonstrate that this granular 

 transformation can be obtained in vitro under the following con- 

 ditions. Prepare a hanging drop with the blood serum of a guinea- 

 pig vaccinated against the cholera vibrio, a serum which has lost the 

 power of transforming, by itself, the vibrios into granules. Add to 

 it a drop of the peritoneal lymph of a normal unvaccinated guinea- 

 pig; this lymph contains dead or living leucocytes and is, by itself, 

 also incapable of producing Pfeiffer's phenomenon. When, to the 

 mixture of these two fluids, which are inactive when they are employed 

 1 Ann. de VInst. Pasteur, Paris, 1895, t. ix, p. 433. 



