212 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



agencies. Natural immunity may be inherited, as when a race of 

 animals shows high resistance to a disease; or acquired during 

 development by any individual having a disease at any period of 

 life. Artificial immunity may be acquired by the various methods 

 of vaccination and inoculation that are common practices in cur- 

 rent medical treatment. For example, in typhoid vaccination 

 the dead germs of the disease are injected into the individual. In 

 response to the stimulus of the organisms introduced during vac- 

 cination, the individual produces substances by his own metabo- 

 lism that protect him against such disease organisms. This is 

 known as the establishment of active artificial immunity because 

 the individual has built up his own protection. As a check against 

 the progress of diseases such as diphtheria, blood serum, pre- 

 pared from an animal, such as a horse, that has been inoculated 

 with the germs, is injected into the patient. In such cases it 

 appears that there is present, in the injected serum, a substance 

 called the antitoxin, which counteracts the toxins or poisons 

 liberated by the bacteria of the disease. This produces passive 

 artificial immunity, so called because the metabolism of the patient 

 takes no part in building up the antitoxin. Immunity thus 

 implies the opposite of " susceptibility " and covers a wide range 

 of phenomena. However, it is to be regarded as but an aspect 

 of the whole series of metabolic events and of the responses of the 

 animal body under stimulation. 



