Investigation as to the causes of immunity and the various methods by which it is produced has not resulted in the discovery of specific methods of treatment for as many diseases as was hoped for at one time. Just at present progress in serum therapy appears to be at a standstill, though vaccines 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 relationships_ 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 necessitates a study of these various features and they will be considered in the discussion to follow. CHAPTER XXVI. THEORIES OF IMMUNITY.