256 THEORIES OF IMMUNITY 



antigen, the substance introduced. Any substance which 

 when introduced into the body causes the formation of an 

 antibody of any kind whatsoever is called an antigen,^ i. e., 

 anti (body) former. 



The foregoing discussion explains Ehrlich's theory of 

 immunity. According to this theory the manner of 

 formation of all antibodies is the same. The kind of anti- 

 body and the manner of its action will differ with the different 

 kinds of antigens used. 



The succeeding chapters discuss some of the kinds of 

 antibodies, the theory of their action and some practical 

 applications. It must be borne in mind throughout the 

 study of these, as has been stated, that every antibody has 

 the property of nnithig with its antigen whether it has any 

 property in addition or 7iot. 



Just what antibodies are chemically has not been deter- 

 mined because no one has as yet succeeded in isolating them 

 chemically pure. To the author they appear to be enzymes. 



Antigens were considered by Ehrlich to be proteins or to 

 be related to proteins. Most workers since Ehrlich have 

 held similar views. Dr. Carl C. Warden, of the University of 

 Michigan, has been doing much work in recent years in which 

 he is attempting to show that the antigens are not proteins 

 but are fats or fatty acids. Mr. E. E. H. Boyer, in his 

 work in the author's laboratory for the degree of Ph.D., 

 received in June, 1920, succeeded in producing various anti- 

 bodies from Escherichia coli antigens. In these antigens he 

 could detect only fatty acids or salts of fatty acids. If the 

 work of these men is confirmed it will open up a most 

 interesting and extremely important field in immunity and 

 in preventive medicine. It is not apparent that the nature 

 of the antigen would affect Ehrlich's theory of the formation 

 of antibodies. 



The author has no doubt that eventually the formation 

 of antibodies and the reactions between them and their 

 antigens will be explained on the basis of physical-chemical 

 laws, but this probably awaits the discovery of their nature. 



^ The term " antigen" is also used to designate substance which may take 

 the place of what are supposed to be the true antigens in certain diagnostic 

 reactions (Chapter XXIX, Complement-fixation Test for Syphilis). 



