IMMUNITY 205 



According to Wright the opsonins are the principal and 

 determining cause of phagocytosis, the act of incorporation by 

 the leucocytes being only the final concluding operation. 

 Opsonins, being the essential factor in immunity, ought not to 

 be present in the serum of fresh animals, i.e., there ought not 

 to be any phagocytosis without opsonins, no spontaneous 

 phagocytosis. But spontaneous phagocytosis is incontestable, 

 as has been established by the experiments of Metchnikoff and 

 Bordet : it is only necessary in in vitro experiments to allow 

 enough time for it to take place : and from the moment that 

 the existence of spontaneous phagocytosis is granted opsonins 

 can no longer play the primary part in the phagocytic 

 process. 



It is certain that the presence of normal serum favours 

 phagocytosis in vitro (Wright and Douglas, etc.) : the serum 

 acts on the bacteria, which are capable of fixing certain of its 

 elements. Are the opsonins substances or properties new and 

 unknown before Wright's researches ? Numerous experiments 

 ascribe to the opsonins of normal serum the same properties as 

 characterize the complement. They are products of the 

 leucocytes. 



In the serum of immunized animals, which favours phagocytosis 

 much more actively than normal serum, the opsonins are not 

 to be distinguished from the immune-body : they can be used 

 for the same re-activation experiments (Levaditi) and they also 

 are products of the phagocytes. 



The bacteriotropins of Neufeld are considered by the 

 majority of workers as being equivalent to the opsonins of 

 immune serum and to the immune-body. 



There is no reason to deny these actions which favour 

 phagocytosis. The work on opsonins and bacteriotropins is 

 simply, to use Ehrlich's expression, a new flowering of the 

 phagocytic doctrine. In Wright's and Neufeld's experiments it 

 is chiefly the experimental method which Metchnikoff criticises. 

 Leucocytes taken from the body, washed and in a different 

 surrounding medium, can no longer accurately represent the 

 phenomena occurring in the living body. The conditions are 



