556 IMMUNITY 



given above. The bacteria in a naturally immune animal, for 

 example, the anthrax bacillus in the tissues of the white rat, are 

 undoubtedly taken up in large numbers and destroyed by the 

 phagocytes, whereas in a susceptible animal this only occurs to 

 a small extent ; and Metchnikoff has shown that they are taken 

 up in a living condition, and are still virulent when tested in a 

 susceptible animal. Variations in phagocytic activity are found 

 to correspond more or less closely with the degree of immunity 

 present, but are probably in themselves -capable of explanation. 

 The fundamental observations of Wright and Douglas show that 

 in many cases at least, leucocytes do not ingest organisms in 

 normal saline solution, and that this is not due to the medium 

 in which they are, is readily shown by subjecting the organisms 

 to the action of fresh serum and then washing them ; thereafter, 

 they are rapidly taken up by the leucocytes in salt solution. 

 In most cases this result is due to the labile opsonin of normal 

 serum which has combining affinities for a great many organisms, 

 as already stated. In other cases more specific substances may 

 be concerned. But the all-important fact is that whether 

 phagocytosis occurs or not, appears to depend upon certain bodies 

 in the serum. As yet we cannot say whether the phagocytosis 

 in a given serum, observed according to the opsonic technique, 

 always runs parallel with phagocytosis in the tissues of the 

 animal from which the serum has been taken. This is a subject 

 on which extended observations are necessary. But whether or 

 not phagocytosis in vivo corresponds with that in vitro it is 

 probably to be explained in the same Avay ; 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. 



(6) 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 



