Immunity against pathogenic micro-organisms 153 



As a matter of fact, it may be stated that the grey rat {Mus 

 decumanus), the black rat (Mus rattus), and white rats are far from 

 enjoying a true immunity from anthrax. They, nevertheless, exhibit 

 a more or less marked resistance against this disease and are always 

 less susceptible than are the other laboratory rodents : mice, guinea- 

 pigs and rabbits. Rats resist attenuated bacilli (anthrax vaccines) 

 better than do these three species, and in order to induce in them 

 fatal anthrax it is necessary to inoculate a much larger number of 

 virulent bacilli. On the other hand, rats are distinguished by a great 

 irregularity in the resistance they offer to the bacillus. At times 

 they resist very virulent bacilli ; at others they contract a fatal 

 disease after an injection of very attenuated bacilli (Pasteur's first 

 vaccine). 



In my first memoir on anthrax^ I noted the fact that in rats the 

 phagocytosis against the bacillus when injected subcutaneously was 

 more marked than after the same inoculation into the rabbit and 

 guinea-pig. Later, this fact was disputed by several observers, who 

 refused to accept the extent and importance of the phagocytic reaction 

 in the rat. This opposition was strengthened by a very interesting 

 discovery made by von Behring^, namely, that the blood serum of 

 the rat possessed a remarkably destructive power for the anthrax 

 bacillus. When this observer added a certain quantity of anthrax 

 bacilli to some blood serum of the rat, instead of elongating into 

 filaments and dividing they underwent a change, lost their normal 

 refraction and took on staining reagents very imperfectly. The 

 membranes alone remained of the bacilli thus treated. Von Behring 

 considered that this bactericidal action of the serum depends on 

 the presence of an organic base dissolved in the blood fluid. He 

 had merely to neutralise the serum by means of an acid, and there 

 was at once a very abundant development of the bacillus. From 

 these researches von Behring came to the conclusion that the 

 natural immunity of the rat from anthrax can be reduced to terms [163] 

 of the chemical action of the blood on the bacillus. 



In one of his most recent publications this author^ returns to the 

 question of anthrax in rats and sums up his present point of view 

 as follows. He regards the immunity of these rodents as being 



1 Virchow's Archiv, Berlin, 1884, Bd. xcvii, S. 516. 



2 CentralU.f. klin. Med,, Bonn, 1888, No. 38. 



3 " Infectionsschutz und Immunitat " in Eulenberg's "Real-Encyclopadie d. ges. 

 Heilkunde," iii*« Aufl. {Encydop. Jahrhucher), Wien, 1900, Bd. ix, S. 196. 



