414 HUMAN BIOLOGY 



in the intestine to fatty acids and glycerine, the proteins to 

 amino-acids and the carbohydrates to simple sugars. 

 Should unaltered fats of foreign origin get into the circulation, 

 abnormal conditions may ensue of which we are more or less 

 in ignorance but which, at any rate, have no bearing on 

 processes of immunity to infection. The same is true of 

 most of the carbohydrates, except certain speciaHzed ones 

 produced by bacteria. In regard to the proteins, however, 

 conditions are different. Complete proteins, including the 

 materials of the bacterial body, form a class of substances 

 which cannot enter the circulation and come into contact 

 with the tissues of the higher animals without arousing 

 reactions by which these tissues are specifically and in all 

 probability permanently changed. For this reason, these 

 materials are grouped together and designated by the word 

 antigen. 



Since the purpose of this chapter is to explain a rather 

 complex state of affairs with as little use of technical phrase- 

 ology as possible, we may be permitted to explain the term 

 "antigen" in greater detail. It is a functional term which, 

 irrespective of chemical structure, designates any substance 

 which can arouse tissue cells to this specific reaction. If 

 materials of this class, whether animal or vegetable proteins, 

 bacteria, or various poisons, or enzymes are administered to 

 the animal body in such a manner that preliminary digestion 

 is avoided, for instance by injection with the hypodermic 

 syringe, there appear after a time, in the circulation of 

 the treated animal, substances which specifically react with 

 the injected "antigen." Almost the entire structure of the 

 science of immunology consists in the detailed elaboration 

 of this simple law of antigens. Thus, to illustrate by an 

 ordinary example, horse serum fed to a normal individual is 

 broken up in the stomach and upper intestine into its 

 component amino-acids, and these are absorbed, distributed 

 and utilized by the cells. The same horse serum, obtaining 

 unchanged access to the circulation, may be demonstrated in 

 the blood, unchanged but gradually diminishing in amount, 

 for a considerable time — days and weeks. As it disappears, 

 however, the blood serum of the injected individual acquires 

 a property not previously possessed, namely, of specifically 



