ACTIVE IMMUNITY 233 



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 adulterations 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 through 

 "immunity reactions" of blood serums have followed from 

 knowledge gained by application 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 neces- 

 sitates a study of these various features and they will be 

 considered in the discussion to follow. 



