154 THEORIES CONCERNING IMMUNITY 



two extremes we shall find all the intermediaries, that is 

 immunity complicated by more or less anaphylaxis. 



We now have sufficient basis to understand the cause of 

 the reactions from antibodies in normal quantity or in excess : 



All antigens are heterologous colloids; colloids can neither 

 be assimilated nor eliminated without having first been 

 changed into salts or into crystalloids. 1 



A rabbit can readily absorb horse serum by mouth without 

 ever becoming ill. The reason for this is that the serum thus 

 absorbed penetrates into the blood only after having been 

 digested. 



On the other hand we know (Hamburger and Moro) that 

 horse serum injected into the blood of a rabbit always finally 

 disappears and we know that it is not eliminated as serum. 



The chemical study of these phenomena is far from com- 

 plete, but judging by what we do know, we can well assume 

 that serum injected into a vein undergoes the same trans- 

 formations in the interior of the organism as in the digestive 

 tract, in other words, that it is digested. This digestion 

 occurs in organs which normally are not adapted to this 

 function. They may be induced to so act by repeated injec- 

 tions of serum, and then produce antibodies in excess which 

 contribute toward this digestion, as characterized by the 

 precipitation of horse serum in vitro and in vivo. 



The antibodies found in sera of immunized animals do 

 not digest their respective antigens in vitro, therefore, they 

 do not contain all the ferments necessary for complete 

 digestion. 



Such antibodies as we have so far been able to obtain 

 in sera must be considered as substances which contribute 

 to this digestion, which prepare for it by fixing themselves 

 to the antigens, and which give in vitro only the first phase 

 of a series of reactions which, effected in the organism, ter- 

 minate by the total disintegration of the albumins. 



1 Certain crystalloids, such as antipyrin, quinine, some mercury salts, 

 become indirect antigens in the organism, either by forming colloidal com- 

 pounds with certain substances of the organism, or by transforming certain 

 substances of the organism, which then become antigens (indirect anaphylaxis 

 of Charles Richet). 



