NATURAL BACTERICIDAL POWERS. 455 



introduced 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 phagocytic 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 insufficient. If such 

 was really the case, these special properties of the phagocytes 

 would demand the same explanation as natural immunity of 

 the individual. While it must be recognised, therefore, that 

 in phagocytic action the body possesses a powerful means 

 of destroying harmless, and, to a certain extent, harmful 

 organisms, and that it is one of the important means by 

 which the bacterium -free condition of the body is main- 

 tained, the facts of natural, just as of acquired, immunity 

 must have another explanation. 



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

 certain 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 experi- 

 ments 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 remarkable immunity 

 to anthrax, had greater bactericidal powers than that of 

 other animals investigated. He found also that the 

 serum of guinea-pigs immunised against the vibrio Metch- 

 nikovi had a bactericidal action, whereas in that of 

 susceptible animals no such action was found. Further 

 investigation, however, has shown that these are not 

 examples of a general law, and that this bactericidal action 

 of the serum does not vary pari passu with immunity either 

 in the natural condition or when artificially produced. 

 The bactericidal action of the serum was specially studied 

 by Buchner and Hankin, who believe that the serum owes 



