428 IMMUNITY. 



2. By injection of the dead organisms. 



3. By injection of filtered bacterial cultures, i.e. toxines ; 



or of chemical substances derived from these. 

 These methods may also be combined in various ways. 



B. Passive Immunity, i.e., produced in one animal by injec- 

 tion of the serum of another animal highly immunised 

 by the methods of A. 



1. By antitoxic serum, i.e., the serum of an animal highly 



immunised against a particular toxine. 



2. By antimicrobic serum, i.e., the serum of an animal 



highly immunised against a particular organism 

 in the living and virulent condition. 



A. Active Immunity. 



\. By Living Cultures. (a] Attenuated. In the earlier 

 work on immunity in the case of anthrax, chicken cholera, 

 swine plague, etc., the methods consisted in the employ- 

 ment of cultures of the living organisms, the virulence of 

 which was so diminished that on inoculation they did not 

 produce a fatal disease, but yet had effects sufficient for 

 protection. The principle is therefore the same as that 

 of vaccination, and the attenuated cultures are often spoken 

 of as vaccines. The virulence of an organism may be 

 diminished in various ways, of which the following examples 

 may be given. 



(i) In the first place, practically every organism when 

 cultivated for some time outside the body, loses its virulence, 

 and in the case of some this is very marked indeed, e.g., the 

 pneumococcus. Pasteur found in the case of chicken 

 cholera, that when cultures were kept for a long time in 

 ordinary conditions, they gradually lost their virulence, and 

 that when sub-cultures were made, the diminished virulence 

 persisted. Such attenuated cultures could be used for pro- 

 tective inoculation. He considered the loss of virulence 

 to be due to the action of the oxygen of the air, as he 

 found that in tubes sealed in the absence of oxygen the 



