THE DEFENSES OF THE BODY 309 



complement is formed, which has the property of inhibiting 

 the action of the complement. 



Tenth, if emulsions of dead bacteria- be injected into 

 animals, the leukocytes of that animal may gain in power 

 to take up and destroy living bacteria of the same species, 

 a result usually attributed to an increase in the opsonizing 

 power of the blood. 



Eleventh, there exists in the blood a body to which Wright 

 has given the name "opsonin," which has the function of 

 so acting upon bacteria that they may be taken up by 

 phagocytes. This preparation of the bacteria by opsonin 

 is regarded as a prerequisite to phagocytosis. 



The foregoing sketch affords but an imperfect idea of the 

 vast amount of labor that has been and continues to be 

 expended upon this many-sided, absorbing topic. Of neces- 

 sity many important contributions have been omitted, but 

 those noted will serve to illustrate the lines along which the 

 solution of the problem has been approached. As a result 

 of such investigations, our knowledge upon infection and 

 immunity may be summarized as follows: 



1. That infection may be considered as a contest between 

 bacteria and living tissues, conducted on the part of the 

 former by means of the poisonous products of their growth, 

 and resisted by the latter through the agency of phago- 

 cytic cells and the proteid bodies normally present in and 

 generated by their integral cells. 



2. That when infection occurs it may be explained either 

 by the excess of vigor of the bacterial products over the 

 antidotal or protective proteids produced by the tissues, or 

 to some cause that has interfered with the normal activity of 

 of the phagocytic cells and production of the protective bodies. 



