THEORIES OF IMMUNITY 19 



conception not only of the mechanism of cure but also that 

 of the pathogenicity. Without denying the possibility of 

 the intervention of leukocytes Ehrlich propounded the idea 

 of indispensable chemical affinities for every reaction between 

 two substances: taking the same point of view for every 

 reaction, whether immunizing or pathogenic, between bac- 

 terial poisons and the cells : namely, that some one substance 

 of the cell possesses a chemical affinity for these poisons. 

 Pathologic symptoms are then the result of fixation of poisons 

 by cellular substance and cure is the result of the multiplica- 

 tion of this cellular substance neutralized by poison, which 

 the cell produces in excess. The product of the combination 

 ought necessarily to be neutral. . 



We shall see further what there is to take or leave in this 

 theory which Ehrlich formulated. It will suffice to note, 

 at the moment, that Ehrlich's theory was no more capable 

 than that of Metchnikoff of explaining the evolution of every 

 disease, and that it could not explain the reaction provoked 

 in the organism by bacterial poisons, or the necessity or 

 possibility of the organism reacting to certain poisons in a 

 way different from that in which it reacts to many other 

 poisons. 



But even if the purely biologic theory of Metchnikoff as 

 well as the purely biochemical theory of Ehrlich cannot 

 explain the entire mechanism of pathogenicity and immunity, 

 yet each contains certain truths which have been the basis 

 of countless studies in the domains of biology, biochemistry 

 and biophysics. These studies have made possible the dis- 

 covery and the definition of a great number of facts concern- 

 ing the properties of pathogenic agents, the nature and the 

 mechanism of their action on the organism, the reactions 

 which they provoke, the nature and properties of the products 

 of their reactions with the fluids of the organism both in 

 vitro and in vivo. 



Notably so, these studies have allowed us to establish a 

 very fine distinction between these pathogenic agents, which 

 are crystallizable and those which are not: these latter being 

 grouped under the general name of colloids. These studies 

 have permitted us to enlarge the horizon, to clarify and to 



