THEORIES OF IMMUNITY 237 



To recapitulate, Ehrlich's theory postulates specific chemi- 

 cal stimuli, which react with specific chemical substances in 

 the body cells, named receptors, and that these receptors, 

 according to Weigert, are produced in excess and hence are 

 excreted from the cell and become free receptors in the blood 

 and lymph. These free receptors are the various kinds of 

 antibodies, the kind depending on the nature of the stimulus, 

 the substance introduced. Any substance which when 

 introduced into the body causes the formation of an anti- 

 body of any kind whatsoever is called an antigen, i. e., anti 

 (body) former. 



If the three fundamental principles just discussed are 

 thoroughly understood, the theory of the formation of 

 different kinds of antibodies should not be difficult to 

 comprehend. 



