594 IMMUNITY 



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 many cases, however, non- 

 pathogenic and also attenuated pathogenic bacteria can be seen to 

 undergo rapid solution and disappear when placed in a drop of 

 normal serum. The bactericidal 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 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 alexins 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. 571), and 

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

 what is seen in the case of an artificially developed hsemolytic 

 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, 

 that toxicity is a relative thing, or, in other words, that different 

 animals have different degrees of resistance or non-susceptibility 

 to toxic bodies. In every case a certain dose must be reached 

 before effects can be observed, and up to that point the animal 



