342 INFECTION AND IMMUNITY. 



true there must exist many kinds of receptors each 

 of which is ahle to unite only with that food sub- 

 stance which has a corresponding binding group of 

 atoms. 



In contrast to the condition with respect to 

 foods, it is held that chemical substances of known 

 composition, drugs and alkaloids never become in- 

 corporated as a part of the protoplasm, that is, 

 they do not unite with cell receptors, although 

 they may affect the vitality and function of proto- 

 plasm profoundly. Their inability to yield anti- 

 bodies as a result of immunization is supposed to 

 depend on this condition. Such substances, ac- 

 cording to Ehrlich, exist in the cell in a condition 

 of unstable salt formation with some constituent 

 of the protoplasm, or in a state of solid solution. 



The following statement from a recent publica- 

 tion by Ehrlich summarizes the nutritional aspect 

 of the theory: "We must assume that all sub- 

 stances which enter into the structure of proto- 

 plasm are fixed chemically by the protoplasm. We 

 have always distinguished between assimilable sub- 

 stances which serve for nutrition and which enter 

 into permanent union with the protoplasm, and 

 those which are foreign to the body. No one be- 

 lieves that quinin and similar substances are as- 

 similated, that is, enter into the composition of the 

 protoplasm. On the other hand, the food sub- 

 stances are bound in the cells, and this union must 

 be considered as chemical. One can not extract a 

 sugar residuum from cells with water, but must 

 first split it off with acids in order to set it free. 

 But now such a chemical union, like every syn- 

 thesis, demands the presence of two binding 



