THE MECHANISM OF IMMUNITY 199 



ease or by vaccination with organisms which produce the disease, 

 or by their products, and (b) passive immunity, by injection of 

 a serum from an actively immunized animal, carrying with it 

 certain substances by which protection is conferred. If 

 organisms are introduced with vaccination, they are rendered 

 comparatively harmless by preliminary treatment. Thus, 

 experience has shown that the organism of smallpox is rendered 

 harmless to man by passing it through the calf, which is only 

 mildly suceptible to the disease. When recovered from the 

 calf, the organisms (virus) , are weakened in such a way that upon 

 inoculation into man they produce only a local disturbance, but 

 enough to change the chemical make-up of the blood, which will 

 then protect the body against smallpox for years. 



A very striking case of passive immunity is furnished by the 

 modern treatment of diphtheria. The ill effects of the disease 

 are due to poisons produced by the parasite of diphtheria 

 these spread through the victim, and by their cumulative effect 

 either cause death or stimulate the cells of the body to produce 

 an antidote in sufficient quantity to neutralize the poison. 

 The actual existence of such an antidote was discovered in 

 1890 by Kitasato and von Behring, and named by them an 

 anti-body. It was found, furthermore, that lower animals 

 could be employed as the source of the anti-body. The horse, 

 for example, may be inoculated with the organisms of diphtheria 

 after some days the blood of the horse contains quantities of 

 the anti-body, so that the serum, if injected into a human vic- 

 tim of diphtheria, counteracts the poison produced by the 

 diphtheria organisms of the victim. It is a case of acquired 

 active immunity in the horse, and acquired passive immunity 

 in the human. 



D. THE MECHANISM OF IMMUNITY 



What is the nature of this change in the blood, whereby 

 organisms or their poisonous products are counteracted? The 

 fundamental principle underlying immunity is that the blood 

 contains something which it did not contain before. Sub- 

 stances which produce this change are called antigens, and 



