582 BACTERIOLOGY. 



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 ineifective, 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 

 of bacteria that in one way or another have been modi- 

 fied. This modification may be artificially produced in 

 the products of virulent organisms, and then introduced 

 into the tissues of the animal ; or the virulent bacteria 

 may be so treated that they are no longer of full viru- 

 lence, and when introduced into the body of the animal 

 will produce poisons of a much less vigorous nature 

 than would otherwise be the case. 



