2- •:*•'•'• '• APPLIED IMMUNOLOGY 



ward off disease varies with different species and 

 among individuals of the same family. Under nor- 

 mal conditions, an animal may be protected indefi- 

 nitely from infection. Nevertheless, if his vital resis- 

 tance be permitted to fall or he be exposed to a viru- 

 lent infection, his defences may crumble instantly and 

 disease be contracted. On the other hand, the natural 

 immunity of certain species to infection is remarkable ; 

 the negro to yellow fever and most of the lower ani- 

 mals to the venereal diseases. 



In this connection allusion should be made to local 

 immunity. By this we mean a natural state of certain 

 organs or tissues, prevalent from birth, due to " in- 

 fective tolerance." For example, the mouth and an- 

 terior urethra normally harbor many different patho- 

 genic bacteria without ill effect, owing to life-long 

 local tolerance with resultant immunity. Introduce 

 some of these germs into the synovial membrane of a 

 joint or into the peritoneal cavity and a virulent in- 

 fection results. Again, the intestinal tract tolerates 

 colon bacilli normally in numbers, which, if access be 

 gained to the urinary tract, may precipitate a grave 

 pathological process. 



Infections common in warm-blooded are rare 

 among cold-blooded animals and vice versa. Verte- 

 brates and invertebrates are not subject to similar in- 

 fections. Field mice are susceptible to glanders. 



