494 IMMUNITY. 



powers of destroying organisms in natural immunity have 

 been ascribed to (a) phagocytosis, and (/;) 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 

 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. The presence or 

 absence of chemiotaxis is also no doubt of importance. 

 But is the phagocytosis the cause or the effect of immunity? 

 The facts of artificial immunity would rather point to its 

 being the latter. The following experiment performed by 

 Metchnikoff, though belonging to the subject of artificial 

 immunity, may be given here. He injected into a guinea- 

 pig a virulent culture of the bacillus of hog-cholera, and at 

 the same time injected the anti-serum of the same organism 

 into a vein. At the end of a few hours a local swelling 

 formed at the site of injection, in which there were enormous 

 numbers of bacilli but no leucocytes. After another injec- 

 tion of the serum, however, the leucocytes gathered around 

 and attacked the bacilli. From this experiment he infers 

 that the serum induced a hyperactivity of phagocytes. 

 The matter, however, may be interpreted from another 

 point of view, namely, that it was not until the toxines of 

 the bacilli were neutralised, or at least till the bacilli were 

 weakened by the action of the serum, that the phagocytes 

 could attack them. All the striking phenomena of phago- 

 cytic action in the case of natural immunity can be looked at 

 from the latter point of view, and it appears to us that the 

 evidence of the essence of natural immunity depending 

 upon special properties of the phagocytes, is quite in- 

 sufficient. Variations in phagocytic activity no doubt play 

 a part, but may themselves be found capable of explanation. 



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

 certain bactericidal powers against different organisms, the 



