METHODS OF PRODUCING ACTIVE IMMUNITY 555 



(2) By passing through the tissues of one species 



of animal (becomes attenuated for another 

 species), t^, « ( -)X Qw —-..- Y 



(3) By growing at alinorfaial temperatures, etc. 



W By growing in the presence of weak antiseptics, 

 or by injecting the latter along with the 

 organism, etc. 



(6) In a virulent condition, in non-lethal doses. 



2. By injection of the dead organisms. 



3. By injection of the dead organisms, " sensitised " by an anti- 



serum. 



4. By injection of filtered bacterial cultures, i.e., toxins ; or of 



substances derived from such filtrates. 

 These methods may also be combined in various ways. 



B. Passive immunity — i.e., produced in one animal by injection 

 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 toxin. 



2. By antibacterial serum, i.e., the serum of an animal highly 



immunised against a particular bacterium in the living 

 and virulent condition. 



Methods of producing Active Immunity. 



1. By Living Cultures. — (a) Attenuated. — In the earlier 

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

 plague, etp., the investigators had to deal with organisms of 

 high virulence, which, had accordingly to be reduced before the 

 organisms could be injected in the living state. It is now found 

 most convenient as a rule to start the process of active immunisa- 

 tion with the injection of dead cultures. The principle is the 

 same as that of vaccination, and both attenuated cultures and 

 also the dead cultures used for injection 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 : — 



(1) In the first place, practically every organism, when culti- 

 vated for some time outside the body, loses its virulence to a 

 greater or less degree, 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 time in ordinary 

 conditions, they gradually lost their virulence, and that when sub- 



