Chapter IV 



TYPES OF IMMUNITY 



Immunity may be defined as non-susceptibil- 

 ity to disease or, as the ability to resist the 

 action of the causes of disease. 



Natural Immunity Most of the known dis- 

 eases attack only certain species of animals; 

 other species, even if artificially inoculated, are 

 insusceptible or naturally immune. Among sus- 

 ceptible species certain families and certain in- 

 dividuals also enjoy immunity. The age of the 

 individual sometimes bestows a degree of im- 

 munity which is more often relative than abso- 

 lute. Such insusceptibility, which is congenital 

 or the result of normal growth, is called "nat- 

 ural immunity. ' ' 



Acquired Immunity Even in susceptible in- 

 dividuals infectious diseases frequently termi- 

 nate in recovery, and persons w r ho have recov- 

 ered are to a greater or less extent resistant to 

 future attacks of the same disease. This condi- 

 tion is called "acquired immunity." 



Acquired immunity may be brought about in 



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