136 THEORIES CONCERNING IMMUNITY 



with those caused by bacteria or bacterial products may be 

 caused by inert substances : identical because they are directly 

 or indirectly antigenic. The introduction of these substances 

 into the organism causes in the organism the formation of 

 more or less specific antibodies , and it has been found that 

 mixtures prepared in vitro of antibodies with their correspond- 

 ing antigens are either neutral or toxic: neutral when they 

 remain clear (toxins and antitoxins); toxic when they pre- 

 cipitate (bacteria or sera and their antibodies) . 



Similar results are obtained in vivo when an animal is 

 injected with an antigen which is precipitated by its serum. 



The necessary conclusions are that : 



1. In those diseases where an antibody is formed which 

 can precipitate its antigen, pathologic manifestations are 

 caused precisely by the very excess of this antibody. 



2. The same pathologic manifestations can be caused by 

 inert antigenic substances. 



3. In many cases the origin of a chronic disease is not the 

 direct action of a toxic substance itself, but a secondary 

 reaction between the organism and an antigen which may in 

 itself be innocuous for example, a harmless bacterium, 

 normal serum, or egg-white. 



4. Finally, in order to obtain an exact and complete idea 

 of the state of an immunized organism, it is not sufficient 

 to consider the accrued resistance to a definite pathogenic 

 agent; but one must also consider the results which this 

 state of immunity, that is to say, the excess of antibody 

 present in the blood and produced by the organs, may have 

 on the nutrition-equilibrium of the immunized organism. 



But a mere statement does not suffice. To convince the 

 reader, it is necessary to produce the "proceedings of the 

 court," and to go over again the stages of the road traveled 

 by experimental biology since the time of Pasteur. 



RESEARCHES BY PASTEUR, BY ROUX AND YERSIN AND 

 BY BEHRING AND KITASATO. 



The researches by Pasteur and by his school have brought 

 out four fundamental biological rules or laws: 



