128 INFECTION AND RESISTANCE 



That the chemical structure of certain bodies determines their 

 ability to enter into relation with cell derivatives such as enzymes 

 is, of course, a fact well established by experiment and explains the 

 specific action of bacterial and other ferments upon certain sub- 

 stances to the exclusion of others. Thus Pasteur noted the fact that 

 bacterial ferments could decompose dextrorotatory tartaric acid 

 while they did not affect the levorotatory variety, and Emil 

 Fischer 33 showed that only those carbohydrates possessing 6 and 9 

 carbon atoms were subject to fermentation by yeasts, and of these 

 only the ones belonging to the "d" series, observations which, by 

 demonstrating the relationship between these active agents of extra- 

 cellular digestion, and the stereochemical configuration of the mole- 

 cules acted upon, lend much support to the logic of Ehrlich's con- 

 tentions. 



Moreover, the recent experiments upon the growth of tissues in 

 plasma outside of the animal body in which cartilage cells produce 

 cartilage, kidney cells, etc., have shown that, given the same nutri- 

 tive materials, the cells themselves must command a certain selective 

 power in the choice of these materials, which can only depend upon 

 a specific element in the structure of the cell receptors. As Fischer 

 has expressed it for fermentation, the ferment must possess an atom 

 group which fits into some group of the fermentable substance as a 

 "key does into a lock,' 7 an analogy which is equally applicable to 

 Ehrlich's conception of the relation of the "side chain" to a nutri- 

 tive molecule. 



Now the toxins and other antigens are, all of them, so far as we 

 know, complex chemical substances, derivatives of animal and vege- 

 table cells, and, for this reason, should have much in common with 

 the materials available for nutrition. It is not strange, therefore, 

 that, coming into contact with the cells of the body during the acci- 

 dents of disease or other abnormal conditions, they should find re- 

 ceptors by means of which they can combine with the cell. Under 

 the ordinary conditions of nutrition a suitable particle taken up by 

 the cell in this way would be assimilated and the receptor either 

 freed for further use or regenerated for the further absorption of 

 similar substances, by virtue of a mechanism delicately coordinated 

 to the needs of cell-nutrition. In the case of the absorption of sub- 

 stances belonging to the class of antigens, however, foreign proteins 

 difficult of assimilation, or of toxins even directly harmful, the re- 

 ceptors occupied by these substances are rendered useless to the cell, 

 and, if the cell continues to live, must be regenerated. If the degree 

 of poisoning or the amount of other antigen introduced has been 

 extremely slight, this regeneration may possibly take place, as in 

 the course of nutritive processes, without further disturbance. If, 

 however, the amounts of antigen are greater than this, or are repeat- 



33 See Oppenheimer, "Die Fermente," Vol. 1. 



