PATHOGENIC BACTERIA 273 



activity of the phagocytes (leucocytes, etc.) in the body are 

 responsible for the immunity. It is very probable that 

 immunity to certain diseases depends on the phagocytes. 



(4) Chemical Theory. Ehrlich and others hold the view 

 that certain chemical substances which are present to a 

 limited extent in all normal serums and cells are increased 

 after having an infection or vaccination and are responsible 

 for both natural and acquired immunity of the body. These 

 substances are bactericidal substances, agglutinins, pre- 

 cipitins, opsonins, etc., and antitoxins. 



Immunity can be most satisfactorily explained on the basis 

 of the phagocytic action of the body cells and the chemical 

 action of certain substances in the body fluids (Nos. (3) and (4). 



