THE SIDE CHAIN THEORY 113 



of the actual requirements, a condition of affairs which the cell meets 

 by throwing off the unnecessary number of receptors as such. These 

 cast-off receptors will, of course, have the same combining groups as 

 the sessile ones, which had originally anchored the toxin molecule, 

 and it stands to reason that if the toxin molecule and the corres- 

 ponding free receptor are brought together either within or outside of 

 the body the two will unite, the result being indicated by absence of 

 toxicity on the part of the mixture. As this is exactly what happens 

 when toxin and the serum of a correspondingly immunized animal 

 are brought together, Ehrlich very properly concludes that the anti- 

 toxic properties of an immune serum are due to the presence of free 

 receptors which are "tuned" to the toxin in question; in other words, 

 that the antitoxin is not newly formed in the body, but identical 

 with those receptors of the cell which render the attack of the toxin 

 upon the cell possible. 



While this conception of the nature, production, and mode of action 

 of the antitoxins originally had reference to these only, subsequent 

 observations led Ehrlich to extend his theory to the other antibodies 

 as well. But in accordance with the facts observed it is necessary 

 to assume that the structure of the other antibodies, viz., those 

 receptors, which enter into relationship with such antigens as the 

 agglutinable substance of bacteria, the precipitable complex of 

 albumins and those cellular constituents which give rise to lysin 

 formation, must be different from that of the antitoxic receptors. 

 For whereas the antitoxic antibodies merely combine with the toxins 

 to form non-toxic components, the other antibodies not only fix the 

 corresponding antigens, but bring about further changes. 



Ehrlich very appropriately remarks that the mere fixation of 

 certain food molecules would not suffice to render them available for 

 purposes of nutrition, but that with molecules of large size, their 

 destruction must precede assimilation. This could be effected, if 

 the receptor in question had not only a haptophoric group "tuned" 

 to the combining group of the food molecule, but in addition, either 

 a second group of ferment character as part and parcel of the same 

 receptor, or a second haptophoric group which might anchor ferment 

 molecules, normally occurring free in the blood, when the other 

 combining group is occupied by a food molecule of a certain structure. 



Experimental investigation has shown that receptors of both type 

 actually exist, and we may accordingly conclude that the antigens 



