498 IMMUNITY. 



by their exerting a positive chemiotaxis on the leucocytes. 

 Variations in phagocytic activity no doubt are found to corre- 

 spond more or less closely with the degree of immunity present. 

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

 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 experiments 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 remark- 

 able 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 pari passu 

 with the degree of immunity. The bactericidal action of the 

 serum was specially studied by Nuttall, and later by Buchner 

 and Hankin, who believe that the serum owes its power to 

 certain substances in it derived from the spleen, lymphatic 

 glands, thymus, and other tissues rich in leucocytes. To 

 these^ substances Buchner gave the name of alexines ; as al- 

 ready explained, they correspond with Metchnikoff's cystases 

 and Ehrlich's complements. These substances are somewhat 

 unstable compounds, and are destroyed by the action of light, 

 and also by a temperature of 6o° C. They can be precipitated 

 by alcohol and by ammonium sulphate, and correspond in their 

 general behaviour with enzymes or unorganised" ferments. Re- 

 garding 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 al- 

 ways 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 considerable, while the animal has no immu- 

 nity. In such a case Metchnikoff says that there occurs in the 

 living body no liberation of alexines by the phagocytes, and 

 hence no bactericidal action such as occurs when the blood is 

 shed. Variations in bactericidal power of the serum as tested 



