6 IMMUNE SERA 



this form of immunity is spoken of as acquired 

 immunity. 



Natural Immunity. It is seldom that natural 

 resistance is absolute. Young animals are often 

 susceptible to an infection against which adults are 

 resistant. Thus young pigeons are readily infected 

 with anthrax while older pigeons are usually 

 refractory. Moreover, the resistance of animals 

 toward infections against which they are relatively 

 immune can often be lowered by artificial means. 

 Frogs can be infected with anthrax if they are kept 

 in water at a temperature of 35 C. Conversely, 

 chickens, which also are relatively immune to 

 anthrax, can be infected if they are chilled. White 

 rats, which are ordinarily resistant to anthrax 

 infection, become susceptible after fatigue or when 

 fed on an exclusively vegetable diet. 



Acquired Immunity. This may be either active 

 or passive, and either form may be acquired naturally 

 or artificially. As examples of naturally acquired 

 active immunity we may mention the immunity 

 developed by one attack of small-pox, scarlet 

 fever, etc. The immunity against small-pox con- 

 ferred by vaccination is an example of artificially 

 acquired active immunity; so is the preventive 

 inoculation with bacterial vaccine against typhoid 

 fever. The best illustration of artificially acquired 

 passive immunity is the injection of diphtheria' 

 antitoxin into humans, while the transmission of 



