Facts bearing on acquired immunity 221 



pigs that are well vaccinated against the cholera vibrio. Under these 

 conditions, as I have demonstrated in my work on the extracellular 

 destruction of cholera vibrios, the vibrios retain their normal form 

 and are never transformed into granules. Pfeiffer has questioned 

 this result, stating that beneath the skin of vaccinated guinea-pigs 

 the granular transformation is always produced, though in a more 

 feeble fashion and after more delay than in the peritoneal cavity. 

 The contradiction between Pfeiffer's experiments and my own can, 

 however, be explained. When inoculating the vibrios into the sub- 

 cutaneous tissue, or during the withdrawal of the exudation formed 

 at the point of infection, small haemorrhages are sometimes produced 

 and a certain amount of microcytase is set free from the leucocytes 

 found in the effusion of blood ; these cells also give up to the extra- 

 vasated blood a portion of their fibrin-ferment. When the experi- 

 ment is successful, that is to say when no haemorrhage is produced 

 during the operations involved, the subcutaneous exudation contains 

 normal vibrios only, without the appearance of any trace of Pfeiffer's 

 phenomenon in the fluid. 



If the extracellular transformation of the vibrios into granules [233] 

 were the real cause of the acquired immunity, the absence of this 

 phenomenon in the subcutaneous tissue of the vaccinated guinea- 

 pig should lead to the death of the animal. As a matter of fact 

 this does not take place and the animal resists the inoculation of the 

 vibrios. This conclusion is open to one serious objection. As the 

 cholera vibrio in the great majority of cases is incapable of producing 

 a fatal infection when inoculated subcutaneously, even in normal 

 unvaccinated guinea-pigs, this example of immum'ty must be placed 

 in the category of natural immunity, a kind of immunity which may de- 

 pend on causes other than those on which acquired immunity depends. 

 To answer this objection it was necessary to select a race of vibrios 

 capable, when injected subcutaneously, of causing death. Mesnil 1 , chief 

 of my laboratory staff, undertook to carry out experiments with the 

 Massowah vibrio, which is regarded by some authors as belonging 

 to the true cholera species. When inoculated subcutaneously into 

 unprotected guinea-pigs, it induces local oedema, in which the vibrios 

 swarm ; the infection rapidly becomes generalised and causes the death 

 of the animal in 24 hours. Yet this vibrio, when injected into the 

 subcutaneous tissue of well vaccinated guinea-pigs, is completely 

 resisted by these animals and not the least trace of Pfeiffer's phe- 

 1 Ann. de Vlnst. Pasteur, Paris, 1896, t x, p. 375. 



