QOfUNITY 245 



pose of securing antitoxins from their blood, as in the prep- 

 aration of diphtheria antitoxin. 



By the injection of antitoxins. These are used to pro- 

 tect against toxins and natural infection, as in the case of 

 diphtheria. 



By the injection of blood serum from immune or hyper- 

 immune animals for preventive purposes. The serum used 

 in the case of hog cholera is an example. 



Active and Passive Immunity. If the organism of the 

 disease is concerned directly in the process of bringing 

 ibout the production of the anti-bodies, the immunity is 

 tinned active. An immunity that is due to recovery from 

 a natural attack is active, as is that produced by injecting 

 the organism or its toxins into the body. Active immunity 

 is slow in its development, is somewhat dangerous, and is 

 always attended with some discomfort to the person or ani- 

 mal in which it is produced. It persists, as a rule, for a 

 considerable period, varying from a few weeks to several 

 years. 



If the blood serum of an animal that has an active im- 

 munity to a disease is injected into an animal that is sus- 

 ceptible to the disease, an immunity is produced which is 

 called passive immunity. It involves no activity of the 

 tissues of the immunized animal. The passively immunized 

 animal is simply the recipient of substances formed in the 

 bodies of other animals and transferred to it. Passive im- 

 munity is rapidly produced, and is attended with little 

 danger and discomfort. The period of protection is meas- 

 ured by a few days or weeks. The most extensive use of 

 passive immunity is in hog cholera, tetanus, and diphtheria. 



It is a well known fact that some outbreaks of a disease 

 are very severe in that many of the infected die, while in 

 another outbreak of the same disease practically all of the 



