CHAPTER V. 

 THE DEFENSIVE FORCES OF THE MACROORGANISM. 



IN the foregoing chapter we have briefly reviewed the aggressive 

 forces of the bacteria and the manner in which they bring about some 

 of the symptoms of the infectious diseases, while we have said nothing 

 as yet of the mechanism by which the macroorganism defends itself 

 against the infection per se, and the action of those poisonous 

 products which are so largely responsible for the clinical picture of the 

 infections. This will be our special problem in the chapters which 

 are now to follow. We may here distinguish between those forces 

 which are at the disposal of the animal body at the moment of infec- 

 tion and those which develop only in the course of the infection, 

 and because of the infection. The former comprise the phagocytic 

 forces of the body cells and the normal bactericidal power of the 

 serum, while the second class includes the various antibodies, so- 

 called, viz., those substances which are liberated from the cells in 

 consequence of the introduction into the circulation of cells or cell 

 products which are foreign to the body. In addition we recognize 

 still other defensive factors, which in a measure are operative in a 

 passive way, but which are nevertheless of great importance from 

 the standpoint of immunity. 



PHAGOCYTOSIS. 



While in the lowest forms of animal life phagocytosis is a sine qua 

 non for the very existence of the individual, representing as it does 

 the only mechanism by which the animal is capable of apprehending 

 its food, insofar at least as this is of an organized type, this property 

 is lost to a greater or less extent as a common cellular characteristic 

 in the higher forms, but is retained by certain cells in all forms of 

 animal life from the lowest invertebrate to the highest vertebrate. 

 In the latter the phagocytic function is, generally speaking, confined 



