198 PUBLIC HEALTH BACTERIOLOGY 



THEORIES OF IMMUNITY. 



The rational explanation of all the phenomena of 

 immunity which are known, is a task yet to be accomplished, 

 It is perhaps better to have a working hypothesis only, 

 as our ideas are being continually enlarged and modified. 

 The many elaborate and complex experiments which have 

 been performed and repeated by many observers are 

 attended by so many consenting circumstances, of some 

 of which we are totally ignorant, that it is not surprising 

 that the inferences from the same experiment are so varied 

 and even at times so conflicting. The words of Pasteur, 

 used in another regard, seem quite appropriate here : 

 " In experimental science, it is always a mistake not to 

 doubt, when facts do not compel affirmation. ... In my 

 opinion the question is whole and untouched by decisive 

 proofs." 



Any theory must take account of phagocytosis, the 

 bactericidal power of normal serum, the results of immun- 

 ization as seen in the formation of antitoxins, bacterio- 

 lysins, agglutinins, precipitins, and opsonins, and any other 

 phenomena which emerge in the further consideration of 

 these. When the theory is built around the phagocyte, 

 it is called a " cellular " theory ; if the body fluids are 

 taken as the key, a " humoral " theory. The final 

 explanation will probably lie in a judicious blending of 

 these two theories. 



Metchnikoff's Phagocytic Theory. In this theory 

 immunization leads to a more rapid and greater leuco- 

 cytosis in response to subsequent infection by the same 

 agent. At the seat of invasion there is also emigration of 

 the microphages from the blood-vessels into the tissues, 

 or if in a serous cavity, the exodus is into the same, giving 

 a cellular exudate. On examination of these cells, many 

 of them are found (in bacterial infections) to contain 

 bacteria in their substance. These are not simply dead 

 bacteria, in process of removal, but living and virulent 

 ones. At a later stage, the bacteria may be seen swollen, 

 granular, and vacuolated, and finally disintegrated. On 

 the other hand, the phagocyte may not be able to digest 

 the engulfed bacteria, and may itself be killed ; and in that 



