Ill 



EHRLICH'S SIDE-CHAIN THEORY 



Our present view of the mechanism of the de- 

 fensive processes of the body against disease is based 

 upon the side-chain theory of Ehrlich. This theory- 

 explains the interaction of the various antigens and 

 antibodies and is almost completely supported by ex- 

 perimental evidence. It is most important to realize 

 that the interaction between antigens and antibodies is 

 of a chemical nature, i.e., the antibody does not destroy 

 the antigen but forms a chemical combination with it. 

 Ehrlich's theory conceives of each body-cell as con- 

 sisting of a central molecular complex or nucleus upon 

 which its life depends and a number of processes or 

 " side chains " capable of combining with foodstuffs 

 for the nutrition of the cell and with foreign substances 

 which might prove injurious to it. These side chains 

 or processes are termed receptors. Each receptor has 

 a special affinity for a certain kind of foodstuff or 

 toxin. Certain receptors are common to all cells, 

 while others are foimd only in special cells. It is also 

 conceivable that some receptors may not be normally 

 present but are formed only by the stimulation of cer- 

 tain forms of toxins. The toxic molecule which unites 

 with the cell receptor consists of two groups, a hapto- 



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