THEORIES OF IMMUNITY 235 



there may be an absence of certain chemical substances 

 in the body cells so that bacteria or their products cannot 

 unite with the cells and hence can do no damage. 



At the present time it is generally accepted, in this coun- 

 try 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. Spe- 

 cific 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 chemi- 

 cal stimuli and thus enable them to act on the cell, which 

 groups Ehrlich has named receptors, and (3) the "over- 

 production" 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, chemical, etc. Chemical stimuli are 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 increase of its corresponding 

 foodstuff. The cells of the body, or certain of them at 

 least, seem to respond in a specific way when proteins or 

 substances closely related to them are brought into direct 

 contact with them, that is, without having been subjected 

 to 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, may be 



