INTRODUCTION 9 



appropriate quantity of such an antigen, after a vari- 

 able interval, during which a leucopaenia may exist, 

 leucocytosis supervenes. After an incubation period 

 of a few days to several weeks, it is found that the 

 body fluids, particularly the blood-serum, react in a 

 novel manner, that is, they possess the property of 

 neutralizing the antigen. Although these are known 

 biological facts, explanations as to when and how these 

 antibodies are formed and when and how they unite 

 with their specific antigens are pure speculation. 

 Nevertheless, immunologists generally have accepted 

 the ingenious so-called side-chain theory of Ehrlich, 

 which conception, although regarded as too visionary 

 by some, has sufficed in a remarkable manner as a 

 working basis for almost all the great problems and 

 discoveries in serology (see Chapter III). On the 

 other hand, JNIetchnikofF's doctrine of phagocytosis 

 (Chapter XVII), not in its original simplicity, plays 

 a not unimportant role in the mechanism particularly 

 of active immunization in view of the researches of the 

 opsonic school. The efficiency of a polynuclear leuco- 

 cytosis in pneumonia has long been recognized as of 

 momentous prognostic value and undoubtedly is inti- 

 mately concerned in the production of immunity. 



The bactericidal and cytolytic substances normally 

 present in the blood-serum and body fluids of some 

 animals, and capable of marked increase by antigenic 



