6 STUDIES IN IMMUNITY. 



When we inject vibrios subcutaneously iii a vaccinated guinea- 

 pig an emigration of leucocytes rapidly takes place. The first white 

 corpuscles to arrive at the point of inoculation find a considerable 

 number of adversaries of which they can destroy only a few. It 

 would be sufficient for a few of these vibrios to be endowed with a 

 slightly more intense power of attraction than their fellows, to cause 

 the phagocytes to direct themselves by preference toward these 

 organisms and to take them up. Very slight, almost inappreciable 

 differences will consequently predestine certain bacteria to rapid 

 phagocytosis. In the same way an inferiority in secretion of toxic 

 products, however slight, will cause a predisposition to rapid 

 destruction. 



In a word, leucocytes kill first those organisms that are less resist- 

 ant, ami the culture inoculated will be freed first of those individuals 

 which cither form less poison than their fellows or attract the 

 leucocytes more. 



In the meantime the vibrios that have been left alone will divide 

 and produce new organisms which in turn will be exposed to the 

 attack of phagocytes. These latter will again suppress those indi- 

 viduals that are most poorly armed for the struggle, and will leave 

 only those that possess in highest degree the two characters just 

 noted. Thanks to this process of selection, new generations of 

 organisms, like those represented by the cultures used in our experi- 

 ments, will be derived for the most part from those bacteria wdiich 

 have been endowed with certain advantages. 



For the production of this new race it is necessary: 



First : that leucocytes come up continually. If they do not come 

 to the point of inoculation, no selection occurs and the organisms do 

 not gain in pathogenicity. So, when virulent vibrios are injected 

 subcutaneously in a normal guinea-pig, the influx of leucocytes is 

 very slight and the organism obtained by this passage has no par- 

 ticular characteristics. If, on the contrary, a normal guinea-pig is 

 given an inoculation of an attenuated vibrio Metchnikovi, which has 

 a strong attraction for leucocytes, the selection is brought about, and 

 the organism increases in virulence. The leucocytes are the active 

 agents in selection, and it is they that, by eliminating the least 

 resistant organisms and sparing the others for a while so that they 

 may multiply, cause the increase in virulence that has been noted. 



