THEORIES OF IMMUNITY. 



danger and practically no discomfort, but is 

 limited in duration and disappears very rapidly. 

 An animal protected against a dose of diphtheria 

 toxine by the injection of antitoxine (of diph- 

 theria) is protected only against that dose of 

 toxine, and is susceptible to a second dose in a 

 very short time. 



Attempts to produce a condition of artificial 

 immunity to infections of various kinds have been 

 made for a very long time, and there exist ac- 

 counts of such attempts among savage tribes — 

 the beginning appears to have been in the 

 direction of protection against the bites of ven- 

 omous serpents. 



The most well-known efforts to secure arti- 

 ficial immunity, and before the present knowl- 

 edge of the bacteria was developed, were the 

 inoculations against small-pox — a knowledge of 

 the beneficial effects of which existed in Asia 

 certainly for generations before its introduction 

 into England by Lady Montague. These inocu- 

 lations consisted in the use of contents of the 

 pustules in mild cases of small-pox, which were 

 introduced through scarifications of the skin or 



