126 INFECTION AND RESISTANCE 



chemically considered as a complex of enormous molecules, must 

 possess a correspondingly great variety of atom groups, by means of 

 which it can unite with these substances to assimilate them and make 

 them a part of its own protoplasm. In order to enter into similar 

 relationship with toxins and other antigens, then, it is only logical 

 to suppose that the cell, in the same way, unites chemically with the 

 antigenic substance, and either assimilates it without sustaining 

 harm, as in the case of non-poisonous complexes, or is injured in the 

 process, as in the case of the poisons. 



The living cell, from this point of view, is conceived as consist- 

 ing of a central chemical nucleus, the "Leistungskern," more or less 

 stable, in that the specialized tissue function is dependent upon it, 

 and a manifold variety of "side chains," or atom groups by means 

 of which it -can enter into relationship with the nutritive and other 

 materials carried to it by the body fluids. The latter term, "side- 

 chains/ 7 is taken from the nomenclature of chemistry, and, although 

 the analogy is a loose one, it serves satisfactorily to elucidate Ehr- 

 lich's meaning. Thus we may conceive the "Leistungskern" as the 

 central carbon ring of any compound of the Benzol series, as, for 

 instance, in salicylic acid in which the hydrogen atoms, the hydroxyl, 



OH OH 



A / 



H C C COOH | |C0 2 CH 8 



H-i i-H V 



\ / 



C Methyl salicylate 



Salicylic acid 

 H 



and the acid radicles represent "side chains." By means of the lat- 

 ter the compound can enter into relation with other substances, as, 

 for instance, with CH 3 in the formation of methyl salicylate. 

 Graphically, though this analogy formulates Ehrlich's fundamental 

 conception, it must not be taken as too literally representing the 

 existing conditions, since, in actual metabolic interchange, there is 

 an infinite variety of possible "side-chain" groups ; for we are deal- 

 ing with an enormous number of assimilable substances, most of 

 them of chemically unknown constitution. The cell, therefore, is 

 looked upon as an active chemical complex, retaining its own: peculiar 

 functional 'characteristics by reason of the "Leistungskern," but 

 constantly getting rid of waste products and entering into new union 

 with extraneous materials by virtue of its "side chains." These side 

 chains, because of their "receiving" function, are spoken of by Ehr- 

 lich as cell "receptors." 



