INTERREACTIONS OF TOXIN AND ANTITOXIN 71 



chemical theory : for they found that if the mixture were allowed 

 to stand for some time at the temperature of the body before being 

 heated, its toxicity was not restored by a temperature of 70 C. 

 This seems to show that the toxin did not exist as such in the 

 mixture, otherwise it would not have been destroyed by the heat ; 

 it must, therefore, have become combined with the antitoxin, or 

 at any rate modified by it in some way. On the other hand, the 

 experiment does not prove that the toxin is completely destroyed 

 beyond all power of further activity ; it simply shows that, when 

 in a condition of combination with its antitoxin, it is less thermo- 

 stable than when free. Similar facts were adduced by Wasser- 

 mann with regard to the combination between pyocyaneus toxin 

 and its antitoxin, and are capable of a similar explanation. 

 Marenghi has also brought forward somewhat similar results 

 with diphtheria toxin. 



Lastly, Ehrlich has shown that the conditions which favour the 

 occurrence of chemical combinations favour the union of toxin 

 and antitoxin e.g., it is accelerated by heat, and takes place more 

 quickly in concentrated than in dilute solutions. 



This brings us to the question as to whether the combination 

 takes place in accordance with the law of multiple proportions 

 a question of great difficulty, but one which has lead in its 

 elucidation to the discovery of facts of much interest. As far as 

 concerns the action of the haemolysins and other toxins that can 

 be readily tested in vitro, there is no doubt that this question, in 

 its simplest form, must be answered in the affirmative. If it 

 requires x c.c. of a given solution of toxin to dissolve exactly 

 i c.c. of a 5 per cent, emulsion of red blood-corpuscles, then it 

 will require 2#, 3*, 4*, etc., c.c. to haemolyze 2, 3, 4, etc., c.c. of 

 the same emulsion. We assume in each case that the haemolysin 

 is added at once, and not in small consecutive amounts. To 

 study the effect of the partial neutralization of toxin by antitoxin 

 we will briefly outline Ehrlich's famous work on the standardiza- 

 tion of diphtheria toxin, and the conclusions he arrived at in 

 consequence of the results thus obtained. 



We have seen that it is possible to determine with a close 

 approach to accuracy the minimal lethal dose of diphtheria toxin 

 for standard guinea-pigs 4.e., those weighing about 250 grammes. 

 This amount is called the toxic unit (TU), and a toxin of which 

 T ^ c.c. is just sufficient to kill a test guinea-pig in three or four 

 days is considered to be normal toxin of unit strength, and is 



