ACTIVE IMMUNITY 251 



diseases the organisms are introduced (c) dead. They are 

 killed by heat or by chemicals, or by using both methods 

 (Chapter XXX). 



When the products of an organism are introduced the 

 resulting immunity is against the products only and not 

 against the organism. If the organism itself is introduced 

 there results an immunity against it and in some cases also 

 against the products, though the latter does not necessarily 

 follow. Hence the immunity may be antibacterial or anti- 

 toxic or both. 



Investigation as to the causes of immunity and the various 

 methods by which it is produced has not resulted in the dis- 

 covery of specific methods of treatment for as many dis- 

 eases as was hoped for at one time. Just at present progress 

 in serum therapy appears to be at a standstill, though vac- 

 cines are giving good results in many instances not believed 

 possible a few years ago. As a consequence workers in all 

 parts of the world are giving more and more attention to 

 the search for specific chemical substances, which will destroy 

 invading parasites and not injure the host (chemotherapy). 

 Nevertheless, in the study of immunity very much of value 

 in the treatment and prevention of disease has been learned. 

 Also much knowledge which is of the greatest use in other 

 lines has been accumulated. Methods of diagnosis of great 

 exactness have resulted, applicable in numerous diseases. 

 Ways of detecting adulteration in foods, particularly foods 

 from animal sources, and of differentiating proteins of varied 

 origin, as well as means of establishing biological relation- 

 ships and differences among groups of animals and determin- 

 ing whether the bloods of two human beings are alike for 

 purposes of transfusion through "immunity reactions" of 

 blood serums have followed from knowledge gained by appli- 

 cation of the facts or the methods of immunity research. 

 Hence the study of "immunity problems" has come to include 

 much more than merely the study of those factors which 

 prevent the development of disease in an animal or result in 

 its spontaneous recovery. A proper understanding of the 

 principles of immunity necessitates a study of these various 

 features and they will be considered in the discussion to 

 follow. 



