THE PROTECTIVE SUBSTANCES OF THE BLOOD. 365 



This, however, is not possible in the case of the substances natu- 

 rally present, and, considering the complicated chemistry of the 

 living organism, we shall probably long continue to be ignorant 

 of the substances which act as the physiological excitants. 



Hence it is not a mere coincidence that the attempt to formu- 

 late a theory for the development of the protective substances suc- 

 ceeded first in connection with those artificially produced. This is 

 now well known as the side-chain or receptor theory. According 

 to my view this theory is also of the highest significance for the con- 

 ception of the nature of the alexins. I shall, however, first outline 

 my views on this subject as they are applied to the formation of 

 antitoxin, as this is comparatively the simplest to study. 



There were, as you all know, chiefly two views concerning the 

 formation of antitoxin, namely, the hypothetical metamorphosis of 

 toxin into antitoxin, and the secretion theory, which approaches 

 somewhat the side-chain standpoint. The former was based on 

 the observation that the antitoxin excited by a certain toxin acts 

 only against just this toxin and against no other. This specific 

 action is such a conspicuous phenomenon that it was at first believed 

 that the intimate relation of toxin to antitoxin could only be explained 

 by assuming the toxin itself to be the mother substance of the anti- 

 toxin. So even to this day, Buchner maintains the view that the 

 antitoxins and related substances do not correspond to preformed 

 or even wholly newly formed constituents of the organism, but that 

 they are non-poisonous transformation products of the substances 

 introduced for purposes of immunization. In this case, therefore, 

 the relationship of antibody to the substances exciting its produc- 

 tion would be due to a similarity of the two components. In other 

 words, there would be no antagonism such as exists between acid and 

 base, but an attraction of like to like, as is seen, for example, in poly- 

 merization, in the attraction of crystallization, or in the structure 

 of starch granules. 



Against this I should like to point out that this assumption can- 

 not apply even from a purely chemical standpoint because the 

 processes advanced as analogous occur in concentrated solutions, 

 while neutralization of toxin and antitoxin takes place in extremely 

 dilute solutions. 



The biological conditions, however, constitute the most serious 

 objection to the assumption of a transformation of toxin into anti- 

 toxin. First comes the enormous difference in quantity which may 



