JNATUKAL JiAUTEKlClDAL POWEES 593 



What may be .called intracellular bactericidal action probably 

 varies in the case of leucocytes of different animals, but regarding 

 .this our knowledge is deficient, and, further, bacteria may some- 

 times survive the cells which have ingested them. In other 

 instances the organisms do not appear to suffer from their 

 intracellular position ; an example of this is afforded in the case 

 of gonococci. 



(6) 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. Further investigation, how- 

 ever, has shown that this is not an example of a general law, 

 and that the bactericidal action of the serum does not vary pari 

 passu with the degree of immunity. In some cases non- 

 pathogenic and also attenuated pathogenic bacteria can be seen to 

 undergo rapid solution and disappear when placed in a drop of 

 normal serum; in the case of many pathogenic organisms, however, 

 the serum has no direct bactericidal effect at all. The bacteri- 

 cidal action of the serum was specially studied by Nuttall, and 

 later by Buchner and Hankin, who believed that the serum owed 

 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 alexins ; as 

 already explained, they correspond with Metchnikoff's cytases 

 and Ehrlich's complements described above. They can be 

 precipitated by alcohol and by ammonium sulphate, and in this 

 respect and in their relative lability correspond with enzymes or 

 unorganised ferments. Variations in bactericidal power of the 

 serum as tested in vitro, however, do not explain the presence 

 or absence of natural immunity against a living bacterium. In 

 some cases, for example, it has been found to be considerable, 

 while the organisms flourish in the body and the animal has no 

 immunity. In such a case Metchnikoff held that there occurs in 

 the living body no liberation of alexins by the phagocyte*, and 

 hence no bactericidal action such as occurs when the blood is 

 shed. In the case of the heemolytic action of a normal serum, 

 it has been shown in many instances that in addition to com- 

 plement a natural immune-body is also concerned (p. 576), and 

 this would appear to be the rule ; the process being analogous to 



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