236 THEORIES OF IMMUNITY 



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

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

 chemical 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 

 the proteins and related bodies, 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 primarily to take up different kinds of food material, 

 Ehrlich calls them receptors. 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. As was stated 

 above, the effect of specific chemical stimuli is to cause the 

 production of more of the particular substance for which it is 

 specific and in the class of bodies under discussion, proteins 

 and their allies, 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 ctually 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. Whenever a cell accumulates 

 an excess of products the normal result is that it excretes 

 them from its own substance into the surrounding lymph, 

 whence they reach the blood stream to be either carried to 

 the true excretory organs, utilized by other cells or remain 

 for a longer or shorter time in the blood. 



