98 INFECTION AND IMMUNITY. 



I. Natural immunity is present in all such cases where, for 

 instance, some species of animals cannot be affected by cer- 

 tain bacteria or their toxins, which are injurious to other 

 species, or, as occasionally happens, when some individuals 

 in a susceptible species are refractive. This does not pre- 

 clude the possibility of such naturally immune animals suc- 

 cumbing to an extraordinarily large dose of the bacteria 

 against which they are immune, or to ordinary doses under 

 extraordinarily unfavorable circumstances. Pasteur has ren- 

 dered naturally immune hens susceptible to anthrax by cool- 

 ing them to a subnormal temperature, and numerous other 

 examples of lowered natural immunity might be cited. 



RACIAL IMMUNITY. There is undoubtedly among certain 

 races a predilection to certain diseases, where, per contra, other 

 races seem to have an immunity. This racial immunity against 

 a special disease seems possibly due to an endemicity of the 

 disease producing what is in reality an acquired immunity. 



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. 



