NATURAL SUSCEPTIBILITY TO TOXINS 557 



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 

 jrttMn with the degree of immunity. In many cases, however, 

 non-pathogenic and also attenuated pathogenic bacteria can be 

 M VM to undergo rapid solution and disappear when placed in a 

 drop of normal serum. The bactericidal action of the serum 

 wus 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 already 

 explained, they correspond with Metchnikoffs cytases and 

 Ehrlich's complements described above. They can be pre- 

 cipitated 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 nourish in the body, and the animal has no 

 immunity. 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. In the case of the hsemolytic 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. 534), and 

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

 what is seen in the case of an artificially developed haemolytic 

 serum. In certain instances an analogous condition appears to 

 obtain in a normal bactericidal serum. For example the dog's 

 serum heated at 58 C. contains a natural immune-body to 

 anthrax which can be activated by the addition of normal 

 guinea-pig's serum so as to produce a bactericidal action, though 

 the latter is by itself without any such effect. At present, how- 

 ever, the possibility of bactericidal action by complement alone 

 cannot be excluded, as it appears to combine with many bacteria 

 without any intermediary. Further work is necessary to deter- 

 mine whether all the facts regarding natural immunity are ex- 

 plainable by the opsonic and bactericidal properties of the serum. 

 2. Variations in Natural Susceptibility to Toxins. We must 

 here start with the fundamental fact, incapable of explanation, 



