SIDE-CHAIN THEORY 



181 



In view of these facts Ehrlich elaborated his " side- 

 chain theory," a theory which, whether it be the real 

 explanation or no, has received a considerable amount of 

 experimental support, and has had far-reaching effects 

 in stimulating research. Ehrlich believed that the 

 chemical activities which are the outcome of *the vital 

 activities of the living cell are due to a complex nucleus 

 or chemical molecule having a ring structure, analogous 

 to the benzene ring, and having attached to it a number 

 of atomic groups or " side-chains." A " side-chain " is 



FIG. 26. Diagram to represent 

 the cell with its various com- 

 bining groups or side -chains. 

 (After Ehrlich.) 



FIG. 27. First stage in anti- 

 toxin formation. Black = 

 toxin molecule. (After Ehr- 

 lich.) 



an atomic group, a carbon atom of which is linked 

 to one of the carbon atoms in a ring. These atomic 

 groups or side-chains are unstable, and enter freely into 

 combination with other groups for which they have an 

 affinity should these be present, and thus the physio- 

 logical activities of the cell, assimilation, nutrition, etc., 

 are carried out (Fig. 26). Now Ehrlich supposed that 

 antitoxin is merely an excess of certain side-chains which 

 are normally present and subserve some of the ordinary 

 functions of the cell and which have become free in the 

 blood, their specific affinities for certain other substances 



