PART TWO 

 CHAPTEE VIII. 



TYPES OF IMMUNITY. 



By immunity we understand that condition in 

 which an individual or a species of animal exhib- 

 its unusual or complete resistance to an infection 

 for which other individuals or other species show 

 a greater or less degree of susceptibility. Immune 

 is from the Latin immunis, which originally ap- 

 plied to one who was exempt from a public service, 

 exempt from tribute, or free. Although the word 

 retained this civil meaning for centuries., and still 

 retains it in certain connections, it also had, even 

 in ancient times, a limited application to the pro- 

 tection which an individual might possess against 

 poisons. It is seen, for example, in descriptions 

 of a tribe inhabiting Northern Africa, the Psylli, 

 who were said to possess a natural immunity to the 

 bites of poisonous snakes. Although we may be 

 certain from this and other references that a con- 

 dition of immunity was recognized in very ancient 

 times, the present significance of the term has 

 developed largely from a better understanding of 

 the nature of infectious diseases and of the condi- 

 tions upon which the resistance of the body de- 

 pends. 



As the definition suggests, we do not think of 



Concerned in. . ., , -. -,- . , ,. -,. 



immunity, immunity to such processes as Bright s disease, 

 arteriosclerosis or the metabolic diseases, but only 



