NATURAL BACTERICIDAL POWERS 499 



the organism. For the study of the various diseases shows that 

 the toxins (in the widest sense) are the weapons by which morbid 

 changes are produced, and that toxin-formation is a property 

 common to all pathogenic bacteria. There is, moreover, no 

 such thing known as a bacterium multiplying in the living tissues 

 without producing local or general changes, though, theoretically, 

 there might be. As a matter of fact, however, natural immunity 

 is in most cases one against infection, i.e. consists in a power 

 possessed by the animal body of destroying the living bacteria 

 when introduced into its tissues : such a 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 Natural 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 (b) 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 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 a 

 neutral 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 



