IMMUNITY AND ITS VARIETIES. 95 



tain bacteria or their toxins, which are injurious to other 

 species, 'or, as occasionally happens, when some individuals 

 in a susceptible species are refractive. 



II. Acquired immunity is manifested when a susceptible 

 animal is protected from the further noxious influences of 

 bacteria either from the fact of having suffered an attack of 

 the disease caused by the bacteria, or when it has been made 

 artificially insusceptible. 



Examples of Natural Immunity. Rats can not be success- 

 fully inoculated with the anthrax bacillus, though other 

 rodents are very susceptible. Again, pigeons are not sus- 

 ceptible but are immune to the anthrax bacillus. The expla- 

 nation of this natural immunity is not easily given. It is 

 supposed in some cases to be due to the mode of living of 

 the immune animal, or to some condition of its secretions, or 

 to some substances found in its blood and tissues which are 

 able to destroy bacterial life or to neutralize their toxins. 

 These substances are called alexins. 



Examples of Acquired Immunity. This may be due, as just 

 mentioned, to a previous attack of disease, and when due to 

 this it lasts in the majority of instances during the life of 

 the animal. In other cases acquired immunity can be arti- 

 ficially induced in animals, and according to the methods 

 used for its production is said to be active or passive. 



1. Active acquired immunity is produced by the action of 

 living germs or their toxins introduced into the animal. 



2. Passive acquired immunity is obtained by a direct trans- 

 ference of an immunizing substance from an immune animal 

 to a susceptible one. Active immunity takes some time to 

 develop, but, as a rule, lasts longer than passive immunity, 

 which is immediately established. 



The Methods of Producing Immunity. 



I. Inoculation, or the introduction of small quantities of live 

 bacteria, so as to produce a mild attack of the disease. This 

 method is dangerous from the fact that it is hard to ascertain 

 how small a quantity of bacteria may be introduced without 



