564 IMMUNITY 



manifestly no physiological action of antitoxin through the 

 medium of the cells of the body can come into play. These 

 facts are practically conclusive in favour of antitoxin action 

 depending upon a direct union of the two substances concerned, 

 and Morgenroth has shown that the combination toxin-antitoxin 

 can be broken up by the action of hydrochloric acid and the 

 two constituents recovered. 



Although authorities are now agreed as to the direct com- 

 bination of toxin and antitoxin, there is still much uncer- 

 tainty as to the exact nature of this union. Regarding this 

 subject there may be said to be three chief views (a) 

 that of Ehrlich, according to which there is a firm chemical 

 union of toxin and antitoxin, and the former is not homo- 

 geneous but has a complex structure ; (6) that of Arrhenius and 

 Madsen, who consider that the phenomena correspond to the 

 behaviour of two substances in weak chemical union ; and (c) that 

 of Bordet, who regards the combination to be not of chemical, 

 but of physical nature, corresponding to a process of adsorption. 

 Controversy on this question may be said to date from the 

 important work of Ehrlich on the neutralisation of diphtheria 

 toxin. Using an immunity unit of antitoxin (the equivalent of 

 100 doses of toxin) he determined with any example of crude 

 toxin the largest amount of toxin which could be neutralised 

 completely, so that no symptoms resulted from an injection of 

 the mixture. This amount he called the limes null dose, ex- 

 pressed as L . He then investigated the effects of adding larger 

 amounts of toxin to the immunity unit and observed the 

 quantity which was first sufficient to produce a fatal result, 

 that is, which contained one M.L.D. of free toxin; this amount 

 he called the limes todtlich, fatal limit, expressed as L t . Now 

 if, as he supposed, the union of toxin and antitoxin resembled 

 that of a strong acid and base, L t - L ought to be the equiva- 

 lent of a minimum lethal dose of the toxin alone. This, how- 

 ever, was never found to be the case, the difference being always 

 considerably more than one M.L.D. For example, in the case 

 of one toxin, M.L.D. = '0165 c.c., L t =l'26 c.c., L ='9 c.c. ; 

 difference = '36 c.c., i.e., 21*9 M.L.D. This, in brief, is what 

 is known as the "Ehrlich phenomenon," and it has been 

 explained by him as the result of the presence of toxoids (vide 

 p. 204), i.e., toxin molecules in which the toxophorous group has 

 become degenerated. He distinguishes three possible varieties 

 of such bodies according to the affinity of the haptophorous 

 group, namely, prototoxoid with more powerful affinity than the 

 toxin molecule, epitoxoid with less powerful affinity, and syntoxoid 



