io THE PRINCIPLES OF IMMUNOLOGY 



tective mechanism of the animal body. The influence of heat on the 

 virulence of organisms is now well known. The degree of heat and 

 the time of exposure must be so adjusted as to reduce virulence 

 without causing actual death of the organisms. Similar reduction 

 of virulence or attenuation may be accomplished by growing the 

 organisms at temperatures which are not optimal. The first ex- 

 ample of this was Pasteur's work in the attenuation of anthrax 

 cultures by growth at 42 to 43 C. The attenuation by means of 

 drying was practised in the classical work of Pasteur on rabies. 

 The virus contained in the spinal cord of rabbits was subjected to 

 desiccation, and it was found that the longer the time of desiccation 

 the less potent was the virus. Chemical agents, such as phenol, 

 acids, iodine and its salts, potassium bichromate, and others may 

 also be used in proper concentrations and for proper periods of time 

 to produce attenuation. Physical agencies, such as growth under 

 pressure, the influence of light, etc., have been employed for pur- 

 poses of attenuation. Of interest in connection with attenuation is 

 the fact that certain organisms, when introduced into the body, vary 

 in virulence, depending upon the route of introduction. For ex- 

 ample, the virus of rabies may be injected intravenously into goats 

 and sheep without producing rabies. This injection, however, serves 

 to confer a certain degree of immunity upon the animals. As has 

 been mentioned before, the organism of cholera may be injected 

 subcutaneously without producing disease, and within certain limi- 

 tations aids in the protection against invasion by these organisms 

 through the intestinal canal. 



