500 IMMUNITY 



not phagocytosis in vivo corresponds with that in vitro it is 

 probably to be explained in the same way ; that is, it probably 

 depends upon the content of the serum. The composition of the 

 latter, no doubt, is the result of cellular activity, and in this 

 the leucocytes themselves are in all probability concerned, but 

 the movements and phagocytic activity of these cells seem to 

 be chiefly if not entirely controlled by their environments. 

 Ingestion is, however, only the first stage in the process ; intra- 

 cellular destruction is the second, and is of equal importance. 

 What may be called intracellular bactericidal action probably 

 varies in the case of leucocytes of different animals, but regarding 

 this our knowledge is deficient, and, further, bacteria may some- 

 times survive the cells which have ingested them. 



(b) When it had been shown that normal serum possessed 

 bactericidal powers against different organisms, the question 

 naturally arose as to whether this bactericidal power varied in 

 different animals in proportion to the natural immunity enjoyed 

 by them. The earlier experiments of Behring appeared to give 

 grounds for the belief that this was the case. He found, for 

 example, that the serum of the white rat, which has a remark- 

 able immunity to anthrax, had greater bactericidal powers than 

 that of other animals investigated. Further investigation, how- 

 ever, has shown that this is not an example of a general law, 

 and that the bactericidal action of the serum does not vary pari 

 passu with the degree of immunity. In many cases, however, 

 non-pathogenic and also attenuated pathogenic bacteria can be 

 seen to undergo rapid solution and disappear when placed in a 

 drop of normal serum. The bactericidal action of the serum 

 was specially studied by Nuttall, and later by Buchner and 

 Hankin, who believe that the serum owes its power to certain 

 substances in it derived from the spleen, lymphatic glands, 

 thymus, and other tissues rich in leucocytes. To these 

 substances Buchner gave the name of alexines as already 

 explained, they correspond with MetchnikofPs cytases and 

 Ehrlich's complements described above. They can be pre- 

 cipitated by alcohol and by ammonium sulphate, and in this 

 respect and in their relative lability correspond with enzymes or 

 unorganised ferments. Variations in bactericidal power of the 

 serum as tested in vitro, however, do not explain the presence 

 or absence of natural immunity against a living bacterium. In 

 some cases, for example, it has been found to be considerable, 

 while the organisms flourish in the body, and the animal has no 

 immunity. In such a case Metchnikoff says that there occurs in 

 the living body no liberation of alexines by the phagocytes, and 



