IMMUNITY 205 



against either toxin or micro-organism. Thus the blood- 

 serum of the fowl, which is highly refractory to tetanus 

 does not exert the slightest antitoxic or neutralising action 

 on tetanus toxin. 



3. Acquired immunity. Acquired immunity may be 

 induced in several ways : 



(1) By an attack of the disease ending in recovery. 



(2) By vaccinating with a modified and less virulent 

 form of the living infective agent (Pasteur's method). 



(3) By treatment with sterilised cultures, or with 

 bacteria-free toxins. 



(4) Occasionally by treatment with sterilised cultures 

 or toxins of a different species. Thus, B. pyocyaneus 

 protects from anthrax (p. 238), and Klein 1 showed that 

 an injection of one of the six following organisms 

 (1) Koch's comma, (2) Finkler-Prior's comma, (3) B. coli, 

 (4) Proteus vulgaris, (5) B. prodigiosus, (6) B. typhosus 

 will protect an animal against any one of the remaining 

 five. He therefore concluded that there is an immunising 

 agent common to all these six organisms, and that this 

 substance is intra- cellular and a constituent of the bacterial 

 cells themselves. In this case, however, the immunity is 

 probably one against certain bacterial proteins and not 

 against the specific endotoxins of the organisms. 



(5) By injection of the blood-serum derived from an 

 animal treated or immunised by method (3) that is to say, 

 antitoxins or other anti-bodies (e.g. amboceptors) are 

 introduced. 



The immunity acquired by methods (l)-(4) is known as 

 " active immunity," because the animal's cells and tissues 

 are altered by the process, so that they are no longer 

 susceptible to the microbe or its toxin. The immunity 

 conveyed by method (5) the injection of an immune 

 serum, is known as " passive immunity," because the 



1 Trans. Path. Soc. Lond., 1893, p. 220. 



