316 INFECTION AND IMMUNITY 



immunity of guinea-pigs to certain strains of proteus, may be cited, 

 for in these cases the leucocyte extract is germicidal, while the serum 

 is not. 



Stated impartially, then, our knowledge of immune bodies and proc- 

 esses stands somewhat thus: There are certain sensitizing substances 

 possibly present in the plasma, certainly in the serum wherever this 

 is formed in a pathological process, which have specific affinity for the 

 invading germs and their products and which when supplied with 

 complement, either normally present in the plasma or derived from the 

 cellular source, may become actively harmful to the bacteria, killing 

 them or even breaking them up, with the liberation of bodies which 

 may become poisonous to the animal per se or by further digestion. 

 This same sensitization by the plasma constituents, while not in itself 

 injuring the bacteria, may subject them to the phagocytic power of 

 the leucocytes. These latter functions of the plasma are spoken of 

 as opsonins or bacteriotropins. 



By a great many workers it is assumed that the opsonic function is 

 carried on by serum constituents entirely separate and apart from the 

 bactericidal or agglutinating functions. The writer is himself inclined 

 to look upon the specific sensitizing body in the serum as probably 

 identical for all these functions, in so far as it concerns one and the 

 same antibody. The antigen is sensitized by the specific antibody and is 

 then rendered subject to the agglutination or precipitation, when 

 physical conditions are suitable, to bactericidal effects in the presence 

 of alexin or to phagocytosis in the presence of leucocytes. 



After phagocytosis has taken place, the germs may be killed and 

 digested. Some of the bactericidal bodies of the phagocytes are sub- 

 stances differing in character from the lytic bodies of the serum, and 

 are either not given off to the serum or are not active in it; whether 

 the lytic amboceptors present in the serum are derived from the leuco- 

 cytes is doubtful and rather improbable. This is supported by the sup- 

 position that guinea-pig leucocyte extracts are not germicidal for 

 cholera and typhoid organisms. Nevertheless, intracellular digestion 

 of these germs does go on; it is possible, therefore, that the ambo- 

 ceptors present in the plasma, whatever their source, attach themselves 

 to the germs and aid in intracellular digestion. 



None of the processes just mentioned leads to the formation of anti- 

 toxins which become free in the plasma or serum. Now, in view of these 

 facts and suppositions, it may possibly be logical to conceive that nearly 

 all pathogenic germs secrete bodies which are not readily soluble in cul- 



