108 



Biological Therapy 



relatively resistant to others. Relatively high resistance, 

 however, may be changed to susceptibility by any devitaliz- 

 ing condition such as shipping, cold, heat or exposure. After 

 this occurs individuals may suffer from disease caused by 

 bacteria to which they are ordinarily extremely resistant. 



IMMUNITY 



Immunity means resistance to disease and is the re- 

 verse of susceptibility. Immunity is divided into natural 

 and acquired, although in any instance immunity is not 

 absolute, but relative. 



NATURAL IMMUNITY 



All species of animals are naturally immune to some 

 disease. Thus, man is naturally immune to hog cholera, 

 while swine are naturally immune to smallpox and diph- 

 theria. Certain individuals are naturally immune to disease 

 to which the majority of the specie are ordinarily suscepti- 

 ble. Thus, many calves are naturally immune to blackleg, 

 although this is a fatal disease to most young cattle. 



ACQUIRED IMMUNITY 



Individuals which are naturally susceptible to certain 

 diseases may develop resistance, or immunity, to these. This 

 is spoken of as acquired immunity and may be acquired 

 either naturally or artificially. Swine which recover from 

 hog cholera and persons who have recovered from smallpox, 

 have developed within their bodies, antibodies which prevent 

 their contracting these diseases a second time. This is 

 referred to as naturally acquired immunity. This same 

 result may be brought about by artificially inducing the pro- 

 duction of antibodies following the injection of an antigen. 

 This is spoken of as artificially acquired immunity and it 

 is upon this type of immunity that the value of biologies 

 depends. This type of immunity is again divided into two 

 kinds, passive and active. 



Passive Acquired Immunity. This is a condition of 

 passive and temporary protection or immunity and may be 

 obtained by the use of any of the immune sera. These sera, 

 as described on "page 111, are made from the blood of horses 

 or cattle which have been previously hyperimmunized 

 (although other animals may be used as in the case of swine 



