116 THE SIDE-CHAIN THEORY 



exclusively, and which accordingly serve the nutrition of the cell, 

 are appropriately termed nutriceptors. 



Under ordinary conditions of cell life, we can well imagine that 

 the cell receptors will have occasion to react only with actual food- 

 stuffs. But we can also conceive that under abnormal conditions, as 

 in the various infections, substances may be brought to the cell which 

 accidentally possess an atomic group that is identical in structure 

 with the combining or haptophoric group, as it is termed, of the 

 usual food molecule to which the special receptor is "tuned." 



To use a homely simile, we may say that while ordinarily only the 

 rightful owner of a house can unlock its doors, the possibility exists 

 that a burglar with a master key could similarly gain entrance; 

 and to carry the simile farther; while the entrance of the house- 

 owner would not be attended by any undesirable consequences, the 

 result might be quite different in the case of the burglar. To return 

 to actual conditions we can readily see that the existence of such a 

 combining group on the part of a toxin molecule, for example, might 

 actually be fatal to the life of the cell, supposing, of course, that the 

 Leistungskern itself could be injured by the toxin. As a matter of 

 fact, this is exactly what Ehrlich supposes to occur in such infections 

 as diphtheria, tetanus, and botulismus. He conceived that the 

 toxic molecule in question must have two distinct molecular groups, 

 one of which the haptophoric group anchors the toxin to the cell 

 receptor, while the other the toxophoric group is the actual bearer 

 of its toxic properties. 



If now we imagine that the number of toxin molecules which have 

 thus gained access to the cell, in virtue of the identity in the struc- 

 ture of its haptophoric group with that of the usual food molecule 

 is not sufficiently large to cause its destruction, the cell would never- 

 theless suffer to a greater or less extent owing to the occupation of 

 important nutriceptors by material that possesses no food value, 

 unless indeed it succeeds in freeing itself of its undesirable encum- 

 brance. When the nutriceptors combine with ordinary food mole- 

 cules we may imagine that this union is not permanent, but that the 

 food molecules are used up chemically to supply the needs of the cell. 

 Coincidently the corresponding receptors are again liberated and 

 placed in a position where they can combine with a new set of food 

 molecules, and so on. If the toxin molecule, on the other hand, 



