INFECTION AND IMMUNITY. 613 



which has the property of inhibiting the action of com- 

 plement. 



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, fascinating topic. 

 Of necessity, 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 at present 

 be summarized as follows : 



1. That infection may be considered as a contest be- 

 tween 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 phagocytic cells and the proteid bodies nor- 

 mally 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 the phagocytic cells and production 

 of the protective bodies. 



3. That in the serum of the normal circulating blood 

 of many animals there exists a substance that is ca- 

 pable, outside of the body, of rendering inert certain 

 pathogenic bacteria, but which is, however, present in 

 such small quantities as to be ineffective, either for the 

 protection of the animal or for the cure of infection when 

 introduced into the body of another animal already in- 

 fected. 



4. That immunity is most frequently seen to follow 

 the introduction into the body of the products of growth 



