THEORIES OF IMMUNITY. 



soluble toxines, that it was the simplest part of 

 the problem. It will, however, by this time 

 appear that the term "simple" is of relative 

 significance only. 



The second group of diseases produced by 

 bacteria attracted attention very soon after the 

 facts in regard to the production of passive im- 

 munity in diphtheria and tetanus were known. 



It was at first supposed that similar antitox- 

 ines could be secured in these diseases also, — 

 such as typhoid fever, cholera, and plague, — but 

 it was quickly demonstrated that this was not the 

 case, except to the very slightest extent. In 

 these diseases, and others in which the bacteria 

 do not produce soluble poisons, the actual pres- 

 ence of the bacteria is necessary for the results 

 to appear, and it soon became evident that the 

 production of immunity in such processes must 

 be dependent to a great degree upon the de- 

 struction of the bacteria themselves rather than 

 to a neutralization of any poisons that they 

 might elaborate. 



65 



