244 SUSCEPTIBILITY AND IMMUNITY. 



products of this microorganism. Although immune against infec- 

 tion by the living microbe, they were killed by the same quantity of 

 a sterilized culture as was fatal to guinea-pigs which had not been 

 rendered immune. 



Charrin has obtained similar results in experiments with filtered 

 cultures of Bacillus pyocyanus. Rabbits which had an artificial 

 immunity against the pathogenic action of the bacillus were killed 

 by doses of a sterilized culture such as were fatal to other rabbits of 

 the same size not immune. In subsequent experiments by Charrin 

 and Gameleia " vaccinated " rabbits were found to be even more 

 susceptible to the toxic action of filtered cultures than were those 

 not vaccinated. Recently (1891) Metschnikoff has followed up this 

 line of experiment, and has shown that when considerable amounts 

 of filtered cultures of Bacillus pyocyanus are injected subcutaneously 

 in rabbits a certain tolerance to the toxic action of the same cul- 

 tures is established in some instances. But his results do not give 

 any substantial support to the view that immunity depends upon an 

 acquired tolerance to the toxic action of the chemical products con- 

 tained in cultures of the pathogenic bacteria with which he experi- 

 mented Bacillus pyocyanus 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- 



