594 IMMUNITY 



what is seen in the case of an artificially developed hemolytic 

 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 Natwral 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 

 has resistance. This natural resistance is found to present very 

 remarkable degrees of variation in different animals. The great 

 resistance of the common fowl to the toxin of the tetanus bacillus 

 may be here mentioned (vide p. 430), and large amounts of this 

 poison can be injected into the scorpion without producing any 

 effects whatever ; the high resistance of the pigeon to morphia 

 is a striking example in the case of vegetable poisons. This 

 variation in resistance to toxins applies also to those which 

 produce local effects, as well as to those which cause symptoms 

 of general poisoning. Instances of this are furnished, for 

 example, by the vegetable poisons ricin and abrin, by the snake 

 poisons, and by bacterial toxins such as that of diphtheria. We 

 must take this natural resistance for granted, though it is 

 possible that ere long it will be explained. 



According to Ehrlich's view of the constitution of toxins, it 

 might be due to the want of combining affinity between the 

 tissue cells and the haptophorous group of the toxin ; or, on the 

 other hand, supposing this affinity to exist, it might be due to 

 an innate non-susceptibility to the action of the toxophorous 

 group. Certain investigations have been made in order to 

 determine the combining affinity of the nervous system of the 

 fowl with tetanus toxin, as compared with that obtaining in a 

 susceptible animal, but the results have been somewhat contra- 

 dictory. Accordingly, a general statement on this point cannot 

 at present be made, though in all probability variations in the 

 susceptibility to the toxophorous group will be found to play a 



