NATURAL BACTERICIDAL POWERS. 4 95 



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 Metchnikovi 

 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 part passu with immunity either in the natural condi- 

 tion or when artificially produced. The bactericidal action 

 of the serum was specially studied by Buchner and Hankin, 

 who believe that the serum owes its power to certain sub- 

 stances in it derived from the spleen, lymphatic glands, 

 thymus, and other tissues rich in leucocytes. To these 

 substances Buchner gave the name of alexines. These 

 substances are somewhat unstable compounds, and are 

 destroyed by the action of light, and also by a temperature 

 of 60 C. They can be precipitated by alcohol and by am- 

 monium sulphate, and correspond in their general behaviour 

 with enzymes or unorganised ferments. Regarding the 

 existence in the serum of bactericidal substances which are 

 very easily destroyed by heat there can be no doubt, but 

 their properties can only be studied outside the body, and 

 it must not be assumed that the serum in such conditions 

 has always the same property as in the living body. In 

 some cases, for example, the bactericidal power of the 

 serum in vitro has been found to be greater than in a 

 living animal. The bactericidal action, moreover, is mani- 

 fested towards some organisms and not towards others, and 

 this variation does not always correspond with the immunity 

 of the animal against these organisms. 



At present, therefore, the facts of natural immunity can- 

 not be fully explained. In some cases the insusceptibility 



