The student will better understand this theory if he recalls his fundamental physiology. Living substance is characterized, among other things, by irritability which is instability. It is in a constant, state of unstable equilibrium. Whenever the equilibrium becomes permanently stable the substance is dead. It is also continually attempting to restore disturbances in its equilibrium. Whenever a chemical substance unites with a chemical substance in the cell, a receptor, the latter is, so far as the cell is concerned, thrown out of function for that cell. The chemical equilibrium of the latter is upset. It attempts to restore this and does so by making a new receptor to take the place of the one thrown out of function. If this process is continued, i.e., if the new receptor is similarly “used up” and others similarly formed are also, then the cell will prepare a supply of these and even an excess, according to Weigert’s theory. Whenever a cell accumulates an excess of products the normal result is that it excretes them from its own substance into the surrounding lymph, whence they reach the blood stream to be either carried to the true excretory organs, utilized by other cells or remain for a longer or shorter time in the blood. Hence the excess of receptors is excreted from the cell that forms them and they become free in the blood. These free receptors are termed antibodies. They are receptors but instead of being retained in the cell are free in solution in the blood. 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.