266 INFECTION AND IMMUNITY 



standardization is still the most useful basis for therapeutic unit 

 determination, the theoretical considerations upon which it is 

 founded can no longer be accepted as conclusive. 



This, however, need not prevent us from discussing the side- 

 chain theory which has no direct relationship to his views on toxins, 

 but offers a very ingenious explanation for the mysterious problem 

 of antibody formation and specificity. 



The Side-Chain Theory. We have seen that the extensive re- 

 searches of Ehrlich into the nature of the toxin-antitoxin reaction 

 led him to believe that the two bodies underwent chemical union, 

 forming a neutral compound. The strictly specific character of such 

 reactions, furthermore, diphtheria antitoxin binding only diphtheria 

 toxin, tetanus antitoxin only tetanus toxin, etc., led him to assume 

 that the chemical affinity between each antibody and its respective 

 antigen depended upon definite atom groups contained in each. 



Ehrlich 23 had, in 1885, published a treatise in which he discussed 

 the manner of cell-nutrition and advanced the opinion that in order 

 to nourish a cell, the nutritive substance must enter directly into 

 chemical combination with some elements of the cell protoplasm. 

 The great number and variety of chemical substances which act as 

 nutriment led him to believe that the highly complex protoplasmic 

 molecules of cells t were made up of a central atom-group (Leistungs- 

 Kern) upon which depended the specialized activities of the cell, 

 and a multiplicity of side chains (a term borrowed from the chem- 

 istry of the benzol group), by means of which the cell entered into 

 chemical relation with food and other substances brought to it by 

 the circulation. If we illustrate graphically by the chemical concep- 

 tion from which the term side chain was borrowed, in salicylic acid, 

 the formula given, the benzol ring represents the "Leistungs-Kern," 



OH 



H C C COOH 



H C C H 



H 



23 Ehrlich, "Das Sauerstoffbediirfniss des Organismus, " Berlin, 1885. 



