IMMUNITY. GENERAL DISCUSSION 123 



been induced by mild infections that have resulted in recovery 

 without the development of definite symptoms of the disease. 

 It is a well-known fact that frequently a small percentage of the 

 hogs in a herd which becomes infected with hog-cholera do not 

 contract the disease, and inoculation experiments show them to be 

 relatively immune. 



Acquired Immunity. Immunity to a disease may be acquired 

 in many different ways. An active acquired immunity is one brought 

 about by the development in the tissues of the animal of certain 

 antibodies which prevent the growth or destroy or neutralize the 

 products of growth of the invading microorganisms. Passive 

 acquired immunity is conferred upon the animal by the injection 

 of antibodies which have been prepared in another animal. A 

 passively immunized animal takes no part in the production of 

 the antibodies to which it owes its immunity. 



Active Acquired Immunity. A development of antibodies 

 and the consequent immunization of an animal may be brought 

 about by the various methods illustrated in the following out- 

 line: 



A. Injection of living microorganisms. 



1. In quantities smaller than the amount needed to produce 



a fatal infection. 



2. Attenuated in various ways 



(a) By growing upon artificial culture-media. 

 (6) By growing at unusual temperatures. 



(c) By heating. 



(d) By growing in the presence of substances inimical to 

 growth, such as weak antiseptics. 



(e) By animal passage. 



B. By injection of dead organisms. 



C. By injection of the products of autolytic digestion of or- 

 ganisms. 



D. By injection of toxins. 



Immunization by the injection of sublethal doses of pathogenic 

 organisms has not been generally practised. With most pathogenic 

 organisms it may be demonstrated experimentally that if the 

 number of living cells injected be decreased below a certain mini- 

 mum, they are no longer capable of producing disease. The method 



