INFECTION AND IMMUNITY 159 



muiiity. Furthermore, it is well established that 

 with certain diseases one attack usually protects 

 the individual for life. This is well seen in 

 small-pox, scarlet fever, and measles. Inasmuch 

 as the individual was previously susceptible, 

 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. 



Ehrlich believes that natural immunity is some- 

 times due to the absence, in the body of the invaded 

 animal, of suitable receptors for the virus. After 

 what has been said in connection with the side 

 chain theory, it is obvious that the virus cannot 

 exert its pathogenic action if there are no receptors 

 whereby it is anchored to the body cells. More- 



