280 Chapter IX 



compelled to recognise that, on the one hand, the protective power of 

 the body fluids may coincide with a susceptibility to the corre- 

 sponding micro-organism, and that, on the other, real acquired 

 immunity may exist without any manifestation of this humoral 

 property, especially as, even in immunised animals, the acquired 

 immunity often persists longer than does this property. It must 

 be accepted then, that, in this immunity, there exists something 

 other than the powers of the fluids of the body, that is to say, the 

 factor which plays the predominant part is to be sought for in the 

 [294] cellular elements. We need only recall the many facts collected in 

 the preceding chapter to be convinced that in acquired immunity 

 phagocytosis is the most constant and most general phenomenon. 

 We find it in cases where the humoral properties are the most 

 marked, as well as in those in which they are only slightly developed 

 or are entirely absent. We need not again discuss Pfeiffer's phe- 

 nomenon analysed in the preceding chapter. It is sufficient to 

 mention that this example of the extracellular destruction of micro- 

 organisms only occurs under limited and special conditions. It is 

 observed only in cases where the injection is made into a situation 

 rich in leucocytes which undergo phagolysis as a result of the sudden 

 change brought about in their conditions of existence. Further, this 

 phenomenon is observed only in connection with micro-organisms 

 that are slightly resistant to the influence of the microcytases. In 

 those cases in which we meet with Pfeiffer's phenomenon, we also 

 meet with a widely extended phagocytic reaction. 



This reaction is most pronounced where the properties of the 

 body fluids are only slightly developed or are absent. The study 

 of acquired immunity against anthrax provides us with a very 

 convincing proof of this. We have already cited the example of 

 vaccinated rabbits and rats in which phagolysis is incomparably 

 greater than in the susceptible control animals which contract a fatal 

 anthrax. This rule is general. It is confirmed in the vaccinated 

 sheep and guinea-pig. The absence, or feeble development, of the 

 protective power of the blood or of the other humoral properties in 

 no way, then, prevents the considerable change which' is set up in the 

 phagocytes of animals that have acquired immunity against anthrax. 

 The negative chemiotaxis of the leucocytes, so marked in susceptible 

 animals, is modified into positive chemiotaxis as the result of 

 vaccination. This fact, one of fundamental importance, was first 

 demonstrated for the immunity against anthrax, later being ex- 



