SUSCEPTIBILITY AND IMMUNITY. 255 



tain tolerance to the toxic action of the same cultures is established 

 in some instances. But his results do not give any substantial sup- 

 port to the view that immunity depends upon an acquired tolerance 

 to the toxic action of the chemical products contained in cultures of 

 the pathogenic bacteria with which he experimented Bacillus pyo- 

 cyaneus and Vibrio Metschnikovi. 



In view of the results of experimental researches above recorded, 

 and of other recent experiments which show that, in certain cases at 

 least, acquired immunity depends upon the formation of an anti- 

 fcoxine in the body of the immune animal, we are convinced that the 

 theory of immunity under discussion, first proposed by the writer in 

 1881, cannot be accepted as a sufficient explanation of the facts in 

 general. At the same time we are inclined to attribute considerable 

 importance to acquired tolerance to the toxic products of pathogenic 

 bacteria as one of the factors by which recovery from an infectious 

 disease is made possible and subsequent immunity established. 



The " vital-resistance theory" of the present writer, as set forth 

 in the above-quoted extracts from his published papers, is essentially 

 the same as that advocated by Buchner at a later date (1883). Buch- 

 ner supposes that during the primary infection, when an animal re- 

 covers, a " reactive change " has been produced in the cells of the 

 body which enables it to protect itself from the pathogenic action 

 of the same microorganism when subsequently introduced. 



Of course when we ascribe immunity to the " vital resistance" of 

 the cellular elements of the body, we have not explained the 

 modus operandi of this vital resistance or " reactive change," but 

 have simply affirmed that the phenomenon in question depends upon 

 some acquired property residing in the living cellular elements of 

 the body. We have suggested that that which has been acquired 

 is a tolerance to the action of the toxic products produced by patho- 

 genic bacteria. But, as already stated, in the light of recent experi- 

 ments this theory now appears to us to be untenable as a general 

 explanation of acquired immunity. 



The Theory of Phagocytosis. The fact that in certain infectious 

 diseases due to bacteria the parasitic invaders, at the point of inocu- 

 lation or in the general blood current, are picked up by the leuco- 

 cytes and in properly stained preparations may be seen in their in- 

 terior, has been known for some years. In mouse septicaemia an 

 infectious disease described by Koch in his work on "Traumatic 

 Infectious Diseases, " published in 1878 the slender bacilli which are 

 the cause of the disease are found in large numbers in the interior of 

 the leucocytes. Koch says, in the work referred to : " Their rela- 

 tion to the white blood corpuscles is peculiar ; they penetrate these 

 and multiply in their interior. One often finds that there is 



