254 THEORIES OF IMMUNITY 



digestion in the alimentary tract, but injected directly into 

 the blood or lymph stream. Cells may be affected by 

 stimuli in one of three ways: If the stimulus is too weak, 

 there is no effect (in reality there is no ''stimulus" acting); 

 if the stimulus is too strong the cell is injured, or may be 

 destroyed ; if the stimulus is of proper amount then it excites 

 the cell to increased activity, and in the case of specific 

 cJiemical stimuli the increased activity, as mentioned for the 

 pancreas, shows itself in an increased production of what- 

 ever is called forth by the chemical stimulus. In the case of 

 many organic chemicals the substances produced by the 

 cells under their direct stimulation are markedly specific 

 for the particular substance introduced. 



2. Since chemical action always implies at least two 

 bodies to react, Ehrlich assumes that in every cell which is 

 affected by a chemical stimulus there must therefore be a 

 chemical group to unite with this stimulus. He further 

 states that there must be as many different kinds of these 

 groups as there are different kinds of chemicals which stimu- 

 late the cell. Since these groups are present in the body 

 cells to take up different kinds of chemical substances, 

 Ehrlich calls them recepttors. Since these groups must be 

 small as compared with the cell as a whole, and must be 

 more or less on the surface and unite readily with chemical 

 substances, he further speaks of them as "side-chains" after 

 the analogy of compounds of the aromatic series especially. 

 The term receptors is now generally used. They are also 

 called haptins. As was stated above, the effect of specific 

 chemical stimidi is to cause the production of more of the 

 particular substance for which it is specific and in the class of 

 bodies under discussion, the particular product is these cell 

 receptors with which the chemical may unite. 



3. Weigert first called attention to the practically con- 

 stant phenomenon that cells ordinarily respond by doing 

 more of a particular response than is actually called for by 

 the stimulus, that there is always an "overproduction" of 

 activity. In the case of chemical stimuli this means an 

 increased production of the specific substance over and above 

 the amount actually needed. 



