558 IMMUNITY 



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. 425), 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 

 very important part. It has been shown by Muir and Browning 

 by means of haemolytic tests that the toxic activity of complement, 

 after it has been fixed to the corpuscles, varies very much ; in some 

 instances an amount of complement, which would rapidly produce 

 complete lysis of one kind of corpuscle, may have practically no 

 effect on another, even though it enters into combination. These 

 results are of importance in demonstrating how the corresponding 

 molecules of different animals may vary in sensitiveness to toxic 

 action. 



Super sensitiveness or Anaphylaxis. 



Under this heading are to be grouped a number of phenomena 

 which in their character and results afford a striking contrast to 

 the state of immunity. The common feature is that repeated in 



