INTRODUCTION 3 



while house mice are immune. Tuberculosis is more 

 prevalent among Jersey than Holstein cattle. Birds 

 and reptiles are not necessarily subject to the same 

 diseases that victimize man. Thus the problem of 

 natural immunity presents many interesting phases, 

 but still lacks an absolutely satisfactory solution. 



Explanations for the existence of natural immu- 

 nity are founded on the protection afforded: first, by 

 the external and internal surfaces of the body ; second, 

 by inflammatory processes ; third, by natural antibac- 

 terial and antitoxic substances, and fourth, by the 

 natural metabolic activity or vital resistance of the 

 organism. 



Bacteria are unable to penetrate the unbroken cu- 

 taneous epithelium, but may reach the subcutaneous tis- 

 sues through abrasions although microscopical in size, 

 through sudoriferous and sebaceous ducts and glands 

 although their secretions are mildly antibacterial, and 

 through the hair follicles. In the subcutaneous tissues, 

 bacterial encroachment is further combated by cellular 

 proliferation and extravasated plasma containing 

 serum, fibrinogen, and leucocytes. Bacteria entering 

 the nasal and oral passages encounter in the mucus 

 and saliva both physical and chemical barriers. The 

 gastric, biliary and pancreatic juices exert antibac- 

 terial and neutralizing functions. 



Inflammation, a manifestation of tissue injury, is 



