IMMUNITY 35 



The intestinal secretions are but mildly preventive of bac- 

 terial growth, but peristalsis aids in dislodgment of micro- 

 organisms. 



Immunity is the ability to resist infection and intoxica- 

 tion. It is always relative and never absolute. 



[ Natural 



Immunity f 



I Acquired ( 



Natural immunity is a natural inherited resistance against 

 infection or intoxication, peculiar to certain groups of animals, 

 but common to all the individuals of these groups. It is 

 peculiar to the kind of animal, not to the individual. Thus 

 the field mouse is susceptible to glanders; the house mouse is 

 slightly immune, and the white mouse is immune. 



Acquired immunity is resistance to infection or intoxica- 

 tion possessed by certain animals of a naturally susceptible 

 kind, in consequence of circumstances peculiar to them as 

 individuals. Active acquired immunity arises from the activ- 

 ities performed by the organism itself. It depends on infec- 

 tion or intoxication, which may have been accidental or 

 intentional; i. e., for the purpose of producing immunity. 

 Some accidental infections, recovery from which renders the 

 individual immune, are measles, scarlet fever, and smallpox. 

 Other infections are followed by an immunity of short dura- 

 tion, as typhoid fever and pneumonia. 



Immunity from intentional infection or intoxication is pro- 

 duced by 04) bringing about a different disease, as in the 

 production of vaccinia to bring about immunity to small- 

 pox. (B) Inoculation with killed bacteria, as in the protec- 

 tive inoculation against typhoid fever or bubonic plague. 

 (C) Inoculation with bacterial products, as diphtheria or tetanus 

 toxin. (D) Inoculation with attenuated cultures of micro- 

 organisms, as in Pasteur's anthrax vaccine or Haffkine's 

 cholera vaccine. (E) Inoculation with virus of increasing viru- 

 lence, as in the protective inoculations against hydrophobia. 



