72 RELATION OF BACTERIA TO DISEASE 



Dr. Ehrlich, a German chemist and physicist. His theory, 

 broadly speaking, assumes a group of substances circulating 

 in the blood which can be stimulated to meet and destroy 

 invaders, and thereby protect the body. Besides the three 

 methods above outlined, in which practical therapeutic use 

 has been made of the known facts in the study of immunity, 

 still others have been devised, but they are scarcely yet out 

 of their experimental stage. 



