THEORIES OF IMMUNITY 253 



or in part neutralize their products; or in some instances 

 there may be an absence of certain chemical substances 

 in the body cells so that bacteria or their products cannot 

 nnite with the cells and hence can do no damage. 



At the present time it is generally accepted, in this 

 country at least, that Ehrlich's theory explains immunity in 

 many diseases as well as many of the phenomena related to 

 immunity, and in other diseases the phagocytes, frequently 

 assisted by chemical substances, are the chief factors. 

 Specific instances are discussed in Pathogenic Bacteriologies 

 which should be consulted. It is essential that the student 

 should be familiar with the basic ideas of the chemical theory, 

 not only from the standpoint of immunity, but also in order 

 to understand the principles of a number of valuable methods 

 of diagnosis. 



The chemical theory rests on three fundamental physi- 

 ological principles: (1) The response of cells to stimuli, in 

 this connection specific chemical stimuli, (2) the presence 

 within cells of specific chemical groups which combine with 

 chemical stimuli and thus enable them to act on the cell, 

 which groups Ehrlich has named receptors, and (3) the 

 ^'overproduction'' activity of cells as announced by Weigert. 



1. That cells respond to stimuli is fundamental in physi- 

 ology. These stimuli may be of many kinds as mechanical, 

 electrical, light, thermal, chemical, etc. The body possesses 

 groups of cells specially developed to receive some of these 

 stimuli— touch cells for mechanical stimuli, retinal cells for 

 light, temperature nerve endings for thermal, olfactory and 

 gustatory cells for certain chemical stimuli. Response to 

 chemical stimuli is well illustrated along the digestive tract. 

 That the chemical stimuli in digestion may be more or less 

 specific is shown by the observed differences in the enzymes 

 of the pancreatic juice dependent on the relative amounts 

 of carbohydrates, fats or proteins in the food, the specific 

 enzyme in each case being increased in the juice with the 

 increase of its corresponding foodstuff. The cells of the 

 body, or certain of them at least, seem to respond in a 

 specific way w^hen substances are brought into direct con- 

 tact with them, that is, without having been subjected to 



