Acquired immunity against micro-organisms 271 



ifluence of protective serums not only on the micro-organisms but 

 io on the organism of the animal into which they are introduced, 

 this influence manifests itself in the form of a strong phagocytosis, 

 is only natural that we should attribute it to the existence of 

 stimulating action of the serums of vaccinated animals on the 

 phagocytes of the normal animals. The detailed analysis of the 



lechanism of the immunity acquired as the result of the injection 

 ^f these serums, as we shall attempt to prove in the following 



lapter, in many cases confirms this view. 

 The important part played by the stimulation of the phagocytic 



taction in acquired immunity is supported by yet another series of 



icts and from a different side. It has been clearly estabUshed that 

 not only the serum of immunised animals but also that of normal 

 man and normal animals, themselves susceptible to the pathogenic 

 action of the micro-organisms, protects the animal organism against 

 infection. This fact was first demonstrated in connection with re- 

 searches on the vaccination of guinea-pigs against the experimental [285] 

 peritonitis produced by the cholera vibrio. 



G. Klemperer^ was the first to observe that the blood of several 

 individuals who had never had cholera was, nevertheless, in the 

 case of guinea-pigs, protective against peritoneal infection by the 

 cholera vibrio. He concluded therefrom that the individuals who 

 had furnished this protective blood possessed immunity against 

 cholera. Soon afterwards I^ was able to extend analogous re- 

 searches over a large number of persons and to show that the 

 protective poAver of the blood is of very wide distribution in human 

 beings. But, instead of assuming that all these individuals, whose 

 fluids protect the guinea-pig from peritoneal infection, possess a 

 natural immunity against cholera, I came to the conclusion that 

 the protective power of the blood cannot be taken as a measure 

 of the immunity of the individual from whom the blood was di-awn. 

 Here again I assumed a stimulant action of the human blood on 

 the phagocytic reaction of the guinea-pig, looking upon it as quite 

 natural that the blood, capable of exciting the reaction in an alien 

 animal, might remain inactive in the body of the animal which 

 furnished it. 



R. Pfeiffer^ has given much attention to the protective action 



1 Berl. Jdin. Wchnschr., 1892, S. 970. 



^ Ann. de Ulnst. Pasteur, Paris, 1893, t. vn, p. 411. 



=* Ztschr.f. Hyy., Leipzig, 1894, Bd. xvi, S. 268. 



