12 INFECTION AND IMMUNITY. 



theories of immunity. The study of opsonins has 

 also served to renew interest in a much-neglected 

 field of specific prevention, namely, bacterial vac- 

 cination,, one of the most gratifying results of 

 which has been a retum to the tuberculin therapy 

 of Koch. 



However, with all our resourcefulness, it is pos- 

 sible that our limitations may soon be reached re- 

 garding the serum and vaccinal therapy of many 

 infections, and we shall be forced to try other prin- 

 ciples of prevention and cure. In connection with 

 this point, the newer chemotherapy, which has 

 been developed in so brilliant a manner by Ehr- 

 lich and others in relation to trypanosomiasis, is 

 of the highest interest. It may be hoped that this 

 work represents only the beginning of a new direc- 

 tion of research, which will be of ultimate value in 

 various bacterial as well as protozoan infections. 



