296 Chapter IX 



acquire the faculty of digesting micro-organisms in a greatly intensi- 

 fied degree. It is with the increase of this digestive power that we 

 have connected the over-production by the phagocytes of the fixative 

 and protective substances which are excreted in large quantities by 

 these cells and pass into the fluids of the animal. As these substances 

 are phagocytic products it may be readily conceived that in certain 

 examples of acquired immunity the animal overcomes the micro- 

 organisms without the protective substances being found in the fluids. 

 It is sufficient that it is in the possession of the phagocytes, which 

 may retain it within themselves and not throw it off into the circu- 

 lation. 



From this account it will be seen that the phenomena, in acquired 

 immunity against micro-organisms, are merely a more or less stereo- 

 typed copy of those that are presented in the animal after the 

 resorption of cells. There, also, we have intracellular digestion with 

 over-production of specific fixatives, part of which are excreted and 

 thus pass into the plasmas. In the resorption of cells there is also a 

 double action of cytases and fixatives ; but in this case the macro- 

 cytases intervene, whilst in the resorption of micro-organisms this 

 function is performed by the microcytases. The fixatives in the two 

 cases are very different from the point of view of their action, for 

 [311] they are specific ; but the cells which act in their production belong, 

 in both cases (resorption of animal cells and of micro-organisms), to 

 the category of phagocytes. 



It is often maintained that the theory I have just summarised is 

 fundamentally opposed to the theory of side-chains or receptors 

 formulated by Ehrlich^. This view I cannot accept. Applied to 

 acquired immunity against micro-organisms this theory may be 

 summed up as follows. The micro-organisms, when inoculated in a 

 non-lethal but immunising dose, combine with certain cells of the 

 animal. The receptors of the micro-organisms find corresponding 

 receptors in these cells, but, when once combined, the receptors of the 

 cells become incapable of fulfilling their normal nutritive function. 

 The cells, thus deprived of their receptors, reproduce such an 

 enormous quantity of them that a portion is excreted into the 

 surrounding medium and passes into the plasmas. These receptors, 

 originating from cells, but which have become constituent parts of 

 the body fluids, are nothing but the fixatives or intermediary bodies, 



1 Ehrlieh, Lazarus u. Piiikus, "Leukaemie, etc." in Nothnagel's "Specielle Pathologie 

 u. Therapie," Wien, 1901, Bd. viii, i Theil, in Heft, Sclilussbetrachtungen, S. 163. 



