THE MECHANISM OF IMMUNITY 195 



are introduced with vaccination they are rendered compara- 

 tively 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 suscepti- 

 ble to the disease. When recovered from the calf the organ- 

 isms (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. 



D. THE MECHANISM OF IMMUNITY 



What is the nature of this change in the blood whereby 

 organisms or their poisonous products are counteracted? A 

 very striking case 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 suffi- 

 cient 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 fur- 

 thermore 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 

 used on a human victim of diphtheria counteracts the poison 

 produced by the diphtheria organisms of the victim. It is a 

 case of acquired immunity. 



The fundamental principle underlying immunity then is that 

 the blood contains something which it did not contain before. 

 Substances which produce this change are called antigens and 

 the new responsive bodies are called anti-bodies. Now the 

 facts of immunity are established and there is an increasing 

 multitude of such facts, but the explanations are purely theo- 

 retical. There are two main hypotheses at the present time 

 which may be in essence the same; one is Metschnikoff's devel- 



