SIDE-CHAIN THEORY 



183 



the action of the toxin, it also must possess two receptive 

 groups or side-chains having an affinity for the hapto- 

 phore and toxophore groups of the toxin ; these may be 

 termed the " receptor " and " toxophile " groups respec- 

 tively (see Fig. 31). The relationship of each fixative 

 group of the corresponding groups viz. that of the toxin 

 and that of the side-chain of the cell must be most 

 intimate, and analogous to the relations to each other of 

 a male and a female screw (Pasteur) or of a lock and its 

 key (E. Fischer), i.e. each must 

 fit or match the other. 



The genesis of antitoxin on 

 the " side-chain theory " takes 

 place in the following manner : 

 Toxin being introduced, the hap- 

 tophore groups of the toxin 

 molecules unite with the par- 

 ticular receptor side-chains of 

 the protoplasm for which they 

 have an affinity (Fig. 27). By 

 this combination the physio- 

 logical activities of the cell are 

 interfered with, a defect is 

 created, the cell is damaged (it is only necessary to 

 consider the case of one cell, or, more strictly, of one 

 molecular group of the cell-protoplasm). But through 

 its recuperative powers the cell soon recovers by the 

 formation of new receptor side-chains to take the place 

 of those which have been put out of action. On 

 injecting more toxin, this combines with these new 

 receptors and a defect is again created (Fig. 28). Once 

 more the cell responds, and a fresh series of receptors 

 is developed (Fig. 29). But by this continual stimula- 

 tion, as it were, the cell commences to form the particular 

 receptors in excess of that needed to repair the defect created, 



FIG. 30. Fourth stage in 

 antitoxin formation. 

 Side-chains, i.e. antitoxin, 

 free in the blood. (After 

 Ehrlich.) 



