280 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



munity which is distinctly temporary. The animal is pro- 

 tected for only a few weeks. By the use of the vaccine, 

 however, the animal may be caused to develop an active 

 immunity which will last for a much longer perior of time, 

 probably for years in many cases. 



Factors Which Determine Immunity. The factors which 

 determine the resistance against disease in plants and ani- 

 mals may be grouped under two general headings: those 

 which are of importance in what may be termed general 

 resistance, and those of importance in specific resistance 

 against particular diseases. 



In animals general resistance is due in large part to the 

 coverings of the body and to the membranes lining its 

 respiratory and digestive tract. The skin is an effectual 

 barrier to the entrance of microorganisms generally. The 

 mucous membrane of the nose and respiratory tract con- 

 stantly throw off mucus upon which dust and microorgan- 

 isms inhaled generally catch, and are pushed up toward the 

 body openings into the throat and nose by the cilia of the 

 epithelial cells. In other words, a cleansing stream of mucus 

 is constantly rising from the lungs and prevents the accu- 

 mulation of dust particles in this organ. In the alimentary 

 tract the digestive juices, particularly the strongly acid 

 gastric juice, destroy many microorganisms. The sub- 

 cutaneous tissues, particularly the layers of fascia, lying 

 parallel to and just under the skin in many parts of the 

 body and separating muscles and subdivisions of tissues 

 from each other, are important agencies in preventing the 

 distribution of microorganisms through the body. 



In plants the general resistance is usually due to the 

 presence of bark or cuticularized layers covering the living 

 tissues. Many plants also contain substances, such as acids, 

 tannins, glucosides, etc., which prevent the development 

 within them of most kinds of microorganisms. 



Specific resistance in animals, as already noted, depends 



