162 . STUDIES IN IMMUNITY. 



It is owing to the presence of these facilitating substances that 

 the sera of vaccinated animals can produce marked evidence of 

 digestion if the cells affected are not too resistant, as, unfortunately, 

 is the case with many bacteria.* We may consider such sera, then, 

 as analogous to digestive secretions. 



This analogy between the properties of active sera and digestive 

 secretions is still more evident when we consider that the active 

 substances in serum arise in the digestive cells that Metchnikoff 

 demonstrated; the function in immunity of such cells is very im- 

 portant and, in the course of evolution, they become identified with 

 those ameboid cells which, in simple organisms, are able to assure 

 the nutrition of the individual, owing to their intracellular digestive 

 functions. It is such cells that, as Metchnikoff has found, take 

 up the function of digestion in slightly differentiated species. What 

 is more, such ameboid cells represent the origin of our digestive 

 apparatus, for, as Metchnikoff has shown, digestion, which at first 

 is only intracellular, becomes, in the course of evolution, extracellu- 

 lar, as these cells acquire the property of excreting their dissolving 

 juices. The source of the agglutinins and the sensitizing substances 

 is not, to be sure, well defined, but, as far as the alexin or dissolving 

 principle is concerned, we know from numerous observations that 

 it is of leucocytic origin. 



In the present state of our knowledge of this as yet obscure 

 subject we do not wish to state too dogmatically such facts as occur 

 to us as correlated. There are at least important characteristics 

 which suggest the relation of active sera to digestive juices, and 

 the analogy would be explicable if, as numerous facts and supposi- 

 tions render it probable, we were to attribute the elaboration of 

 these active substances to the phagocytes, in other words, to that 

 group of cells endowed with digestive properties which are retained 

 during evolution. 



If future facts point to the same conclusions, we shall finally 

 regard immunity, not only from the biological standpoint, as we do 

 at the present moment, but also from the chemical standpoint, as an 

 instance of the physiology of digestion. 



* As we know, the typhoid bacillus and the colon bacillus show only slight 

 granular transformation in the presence of an active serum. Many other bacteria 

 are still less susceptible to bactericidal properties. 



