464 IMMUNITY. 



1. By injection of the living organisms. 



(a) Attenuated in various ways. Examples : — 



(i) By growing in the presence of oxygen, or in a 



current of air. 

 (2) By passing through the tissues of one species 



of animal (becomes attenuated for another 



species). 

 '(3) By growing at abnormal temperatures, etc. 

 (4) By growing in the presence of weak antiseptics, or 



by injecting the latter along with the organism, 



etc. 

 {b) In a virulent condition, in non-lethal doses. 



2. By injection of the dead organisms. 



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



A. Active Im^nunity. 



I. 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 employment 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 culti- 

 vated for some time outside the body, loses its virulence, and 

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

 coccus. Pasteur found in the case of chicken cholera, that 



