192 A MANUAL OF BACTERIOLOGY 



sidered to be between two substances only, toxin and 

 antitoxin, that it is reversible, and that when the system 

 has reached equilibrium, a fraction of toxin and also of 

 antitoxin remain free, this fraction of toxin producing 

 the '*' toxone effect " (see p. 194). If equivalent quan- 

 tities of acetic acid and alcohol are mixed, the reaction 

 is never complete ; the acid and alcohol never entirely 

 disappear, because the water formed reacts with the ethyl 

 acetate, reconverting it into acid and alcohol. Such a 

 reaction is termed reversible, and this particular case 

 could be thus represented : 



CH 3 .COOH + C 2 H 5 .OH ^ CH 3 .COOC 2 H 5 + H 2 0. 



Bordet has suggested that the fixation of toxin by 

 antitoxin is an adsorption phenomenon, similar to the 

 fixation of a dye by a tissue. 



These hypotheses may now be examined more in detail. 

 Ehrlich's experiments 1 on diphtheria toxin seemed to 

 show that the neutralisation of toxin by antitoxin follows 

 the laws of simple chemical combinations, such as the 

 neutralisation of a strong base (NaOH) by a strong acid 

 (HC1). If so, it would be expected that antitoxin would 

 neutralise proportionate amounts of toxin ; but this is not 

 so. and Ehrlich was forced to the conclusion that toxin 

 is a complex mixture of proto-, deutero-, and trito-toxin, 

 and toxone, with different toxicities and different avidities 

 for antitoxin. Moreover, when toxin is kept it decreases 

 in toxicity, though still retaining much of its avidity 

 for antitoxin. Ehrlich assumed, therefore, that the toxin 

 becomes transformed into substances termed toxoids, 

 which are non-toxic but retain their affinity for antitoxin 

 (see also section on the standardisation of diphtheria 

 antitoxin). This he explained as due to destruction of 

 the unstable toxophore groups, with the retention of the 



1 See Croonian Led. Roy. Soc. Lond., 1900 ; also p. 336. 



