One function of the free receptor, the antibody, is _always to unite with the chemical substance which caused it to be formed_. _It may have additional functions._ The chemical substance which caused the excess formation of receptors, antibodies, is termed an _antigen_ for that particular kind of antibody. To recapitulate, Ehrlich’s theory postulates _specific chemical 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, 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_,[23] _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 antibody_ 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 uniting with its antigen whether it has any property in addition or not_. Just what antibodies are chemically has not been determined 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 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 (not yet published) in the author’s laboratory for the degree of Ph.D., received in June, 1920, succeeded in producing various antibodies from _Bacterium 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. CHAPTER XXVII. RECEPTORS OF THE FIRST ORDER. ANTITOXINS--ANTIENZYMES.