526 IMMUNITY 



developed in the blood of the highly immunised animals. A 

 corresponding antagonistic body, to which Eraser gave the name 

 " antivenin," appears in the blood of animals in the process of 

 immunisation against snake poison. 



These investigations are specially instructive, as such vegetable 

 and animal poisons, both as regards their local action and the 

 general toxic phenomena produced by them, present, as we have 

 seen, an analogy to various toxins of bacteria. 



Nature of Antitoxic Action. This subject is only part of the 

 general question with regard to the relation of anti-substances 

 to their corresponding antigens, but it is with regard to anti- 

 toxic action that most of the work has been done. We have to 

 consider here two points, namely, (a) the relation of antitoxin 

 to toxin, and (b) the source of the antitoxin. With regard to the 

 former subject there is now no doubt that the antagonism 

 between toxin and antitoxin is not a physiological one but that 

 the two bodies unite in vitro to form a compound inert towards 

 the living tissues, there being in the toxin molecule an atom 

 group which has a specific affinity for the antitoxin molecule or 

 part of it. We shall consider the facts in favour of this view, 

 and in doing so we must also take into account the anti-sera of 

 the vegetable toxins, of snake poisons, etc. 



When toxin and antitoxin are brought together in vitro, it 

 can be proved that their behaviour towards each other resembles 

 what is observed in chemical union. Thus it has been found 

 that a definite period of time elapses before the neutralisation 

 of the toxin is complete, that neutralisation takes place more 

 rapidly in strong solutions than in weak, and that it is hastened 

 by warmth and delayed by cold. C. J. Martin and Cherry, and 

 also Brodie, have shown, that in the case of diphtheria toxin and 

 in that of an Australian snake poison the toxin molecules will 

 pass through a colloid membrane (p. 193), whilst those of the 

 corresponding antitoxin will not. Now, if a mixture of equivalent 

 parts of toxin and antitoxin is freshly prepared and at once 

 filtered, a certain amount of toxin will pass through, but the 

 longer such a mixture is allowed to stand before filtration the 

 less toxin passes, till a time is reached when no toxin is found 

 in the filtrate. Further, if the portion of fluid which at this 

 stage has not passed through the filter be injected into an animal 

 no symptoms take place ; this shows that after a time neutral- 

 isation is complete. Again, in cases where the toxin has some 

 definite physical effect, demonstrable in vitro, e.g. lysis, aggluti- 

 nation, coagulation, or the prevention of coagulation, its action 

 can be annulled by the antitoxin ; in such circumstances 



