CELLULAR THEORY OF IMMUNITY 319 



One might have understood a conflict between the 

 microbe and the cells directly reached by the inoculation 

 or located in its neighborhood; but obedient to what 

 mysterious call do the white cells come from all parts of 

 the organism, filtering through vessels, and penetrating 

 to the region where they will be useful? The living cells 

 have no emotion, not even that of well-being, they have 

 only needs, and obey only physical or chemical actions. 



The discovery of chemiotaxis, and the extension to the 

 leucocytes of ideas introduced into science by Pfeiffer 

 has taken away from the theory of M. Metchnikoff a 

 little of its mysterious aspect, and with the same stroke 

 has brought back to the field of chemistry the question 

 which had been referred to the cellular field. It dem- 

 onstrates the existence in the leucocyte of a sort of 

 far-away scent, which indicates to it the directions in 

 which it will find substances suited to its taste, or from 

 which it can derive benefit. These substances are se- 

 creted by the microbes used for inoculation, or intro- 

 duced with them in the bouillon cultures. Immediately, 

 they challenge the enemy and the struggle begins. It 

 can happen, and in fact does happen sometimes that the 

 secretions of the bacillus are not inciting, and even that 

 they are repellent. Then the bacillus protects itself 

 against the leucocytes, and can develop at its ease, if the 

 host does not put into play secondary causes of resistance. 



As to the struggle, when once begun, its issue always 

 rests undecided a priori. Sometimes the leucocyte, 

 surrounds the microbe and digests it. It becomes a 

 phagocyte. Sometimes also the ingested microbe suc- 

 ceeds in remaining alive, continues to secrete injurious 

 substances, a toxine, and it is the leucocyte which suc- 

 cumbs. In cases in which disease follows the inocula- 

 tion, the victory remains undecided for some time, then 

 results in favor of one of the adversaries. 



