THEORIES OF IMMUNITY. 



filtered culture fluids in which bacteria have 

 grown and produced soluble poisonous products. 

 Examples of the first method are the proced- 

 ures in securing immunity against typhoid fever, 

 cholera, and plague; of the second, the first and 

 second " vaccines " of anthrax ; of the third, the 

 use of the soluble toxines of tetanus and diph- 

 theria. 



Passive acquired immunity. — It has been 

 found that in certain cases of active immunity 

 produced as above, there appears coincidently 

 with the complete resistance to the injection of 

 the toxic material a substance, existing in the 

 blood-serum of the animals so treated, that has 

 an antagonistic action to the toxic material, both 

 in the test-tube and in the living animal tissues. 

 This substance is spoken of as " antitoxine," and 

 with the serum containing it is used for the pro- 

 tection of tissues invaded by the toxine-producing 

 bacteria; as in the therapeutic use of diphtheria 

 antitoxine. 



Such antitoxines occur in the blood-serum of 

 animals attacked by, or immunized with the toxic 

 products of tetanus and diphtheria — the cause 



