592 IMMUNITY 



power may exist though the animal is still susceptible to the 

 separated toxins. We shall now look at these two factors 

 separately. , 



1. Variations in Natwal Bactericidal Powers. — The funda- 

 mental fact here is that a given bacterium may be rapidly 

 destroyed in one animal, whereas in another it may rapidly 

 multiply and produce morbid effects. The special powers of 

 destroying organisms in natural immunity have been ascribed 

 to (a) phagocytosis, and (6) the action of the serum. 



(a) The chief factors with regard to phagocytosis have been 

 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 showed 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 to any 

 extent 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 sub- 

 stances 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 way ; that is, 

 it probably depends upon the content of the serum. The com- 

 position of the latter, no doubt, is the result of cellular activity, 

 and in this the leucocytes themselves are in all probability con- 

 cerned, 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. 



