Immunity against micro-organisms 309 



bacilli had become permeated with fixatives, but, in spite of this, 

 they killed the mice almost as quickly as the controls. Consequently, 

 it is not the fixative adherent to the micro-organisms which deter- 

 mines the protective action of the specific serum. This fluid must 

 contain another substance, one that will stimulate the phagocytes. 



The analysis of the mechanism of the immunity termed passive, that 

 is to say, communicated to normal animals by the introduction of an anti- 

 infective specific serum, teaches us that, even when the direct action of 

 the humoral substances is very limited, the protective effect, thanks to 

 the stimulant action which brings about the destruction of the micro- 

 organisms through the mediation of the phagocytic reaction, is still 

 produced. The result at which we have thus arrived is confirmed by 

 the examination of the phenomena observed in animals subjected to 

 the action of anti-anthrax serum. Marchoux 1 first supplied us with 

 precise details as to the mode of action on the rabbit of the serum of 

 animals treated with anthrax bacilli. He found that, in the peritoneal 

 cavity of rabbits injected the day before with anti-anthrax serum, the 

 inoculated anthrax bacilli almost immediately become the prey of [325] 

 phagocytes. Within a couple of minutes after the introduction of 

 bacilli into the peritoneal cavity, the great majority of them are 

 ingested by the leucocytes ; ten minutes later, there are no free 

 bacilli. Not only the ingestion but also the destruction of these 

 micro-organisms takes place with great rapidity, and even a few hours 

 after the injection, the peritoneal exudation, when sown on nutrient 

 media, remains sterile. In the subcutaneous tissue the phagocytic 

 reaction requires a longer time than in the peritoneal cavity, never- 

 theless, it goes on very rapidly. Thus, when inoculated into the 

 subcutaneous tissue of the ear of rabbits treated with specific serum, 

 the bacilli are in great part ingested at the end of half-au-hour. At 

 the end of an hour phagocytosis is usually complete. 



In Marchoux's experiments the importance of the part played by 

 the phagocytes becomes still more prominent when it impedes their 

 function in any way. Rabbits injected with anti-anthrax blood and 

 24 hours later inoculated below the skin of the ear with anthrax 

 bacilli always resist infection, exhibiting the well-marked phago- 

 cytosis just mentioned. In other rabbits, however, prepared in the 

 same way with the serum, but inoculated the following day into an 

 ecchymosis excited by tapping the ear lightly, a certain number of 

 the bacilli escape the phagocytes and succeed in setting up an 

 1 Ann. de FInst. Pasteur, Paris, 1895, t. K, p. 800. 



