474 IMMUNITY. 



Nature of Antitoxic Action. — We have to consider here two 

 points, viz., {a) the relation of antitoxin to toxin, and (b) the 

 source of the antitoxin. With regard to the former subject there 

 has been much diversity of opinion. Some observers consider 

 that the antagonism between toxin and antitoxin depends upon 

 a chemical^ union between the two substances, whilst others con- 

 sider that it is of a physiological nature, the antitoxin acting 

 through the medium of the cells of the organism. The bulk of 

 evidence recently brought forward is, however, strongly in favour 

 of the view 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 a simple chemical union. It is of course to 

 be kept in view that the only test of the neutralisation of the 

 toxin by the antitoxin is that when the resultant mixture is 

 injected into a susceptible animal no symptoms occur. As in 

 chemical union, a definite period of time elapses before the 

 neutralisation of the toxin is complete. Other points of resem- 

 blance to simple chemical union are found in the facts that 

 neutralisation takes place /nore rapidly in strong solutions than 

 in weak, and that it is hastened by warmth and delayed by cold. 

 It has been found that if these factors be taken into account 

 . and a standard toxin of definite strength be employed, a toxin 

 can be titrated against an antitoxin with corresponding accuracy 

 to what obtains in the case of an acid and an alkali. 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. 174), 

 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 mixtures are 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 



