Immunity against pathogenic, micro-organisms 167 



and continue to multiply. This experiment, which readily succeeds, 

 demonstrates very clearly the difference between the positive sus- [177] 

 ceptibility of the leucocytes (with respect to the Proteus) and the 

 negative (with respect to the streptococcus). Bordet, in accordance 

 with the view now generally accepted, regards this sensitiveness as a 

 chemiotaxis, that is to say a perception of the chemical composition 

 of the surrounding medium. It must be admitted that the substance 

 which excites the chemiotaxis of the leucocytes does not readily 

 diffuse and may not, therefore, be found in a state of solution in the 

 plasma of the peritoneal exudation. Otherwise the leucocytes would 

 refuse to ingest, not only the streptococci, but also the small Proteus 

 bacilli, bathed in the same repellent fluid. It is more probable that 

 the substance which excites the negative chemiotaxis is contained in 

 the aureola that surrounds the streptococci, from which it only 

 escapes with difficulty and for a short distance. 



Marchand 1 continued the investigation of the same subject in 

 Denys' laboratory at Louvain. He studied the natural resistance of 

 the guinea-pig, rabbit and dog against the streptococcus. He, also, 

 came to the conclusion that phagocytosis constitutes the principal 

 means of defence of these mammals in their struggle against one of 

 the most formidable of the pathogenic micro-organisms. Starting 

 from a single colony, Marchand obtained two distinct races, one very 

 virulent for the rabbit, the other encountering a most effective 

 natural resistance. This resistance is due to the activity of the 

 phagocytes which destroy the streptococci in the ordinary fashion. 

 He states as the general result of his investigation that "an at- 

 tenuated streptococcus is a streptococcus readily devoured by phago- 

 cytes " whilst " a very virulent streptococcus is a microbe that is not 

 attacked by the leucocytes," and he adds that "a streptococcus is 

 virulent because it is not devoured by phagocytes " (I.e. p. 270). Up 

 to this point the views of Marchand are in accord with those of 

 Bordet ; but here they diverge, in fact as soon as it becomes a 

 question of the explanation of the origin of the difference in the 

 behaviour of the leucocytes. Marchand refuses to apply the theory 

 of chemiotaxis and asserts " that the phagocytosis depends on some 

 physical property of the streptococcus and is consequently depen- 

 dent on the tactile functions of the leucocytes" (p. 292). The 

 experiments upon which he founds his conclusion cannot, however, 



1 Arch, de med. exper. et d'anat. path., Paris, 1898, t. x, p. 253. 



