464 IMMUNITY. 



of vaccination against smallpox depends upon the same 

 principle. 



(3) Many organisms become diminished in virulence 

 when grown at an abnormally high temperature. The 

 method of Pasteur, already described (p. 313), for producing 

 immunity in sheep against anthrax bacilli, depends upon 

 this fact. A virulent organism may also be attenuated 

 by being exposed to an elevated temperature which is 

 insufficient to kill it. Toussaint at an early date obtained 

 protective inoculation against anthrax by means of cultures 

 which had been exposed for a certain time to a temperature 

 of 55 C., though it is possible that in some cases the 

 bacilli were really killed, and immunity resulted from their 

 chemical products. 



(4) Still another method may be mentioned, namely, the 

 attenuation of the virulence by growing the organism in the 

 presence of weak antiseptics. Chamberland and Roux, for 

 example, succeeded in attenuating the anthrax bacillus by 

 growing it in a medium containing carbolic acid in the 

 proportion of i : 600. The virulence may also sometimes 

 be attenuated by injecting certain chemical substances along 

 with the bacteria into the body. Iodine terchloride was 

 found by Behring to modify in this way the virulence of 

 the diphtheria bacillus. 



These examples will serve to show the principles under- 

 lying attenuation of the virulence of an organism. There 

 are, however, still other methods, most of which consist in 

 growing the organism in conditions somewhat unfavourable 

 to its growth, e.g., under compressed air, etc. 



(b) By living Virulent Cultures in non-lethal Doses. 

 Immunity may also be produced by employing virulent 

 cultures in small, that is, non-lethal doses. In subsequent 

 inoculations the doses may be increased in amount. For 

 example, immunity may thus be obtained in rabbits against 

 the bacillus pyocyaneus. Such a method, however, has 

 had a limited application in the case of virulent organisms, 

 as it has been found more convenient to commence the 

 process by attenuated cultures. 



