558 Chapter XVII 



chains or receptors, produced in excess and expelled into the blood 

 [582] plasma by the cells which produce them. Ehrlich does not tell us to 

 what category these cells belong ; he maintains only that these cells 

 must be in possession of receptors endowed with a specific affinity 

 for certain molecular groups of micro-organisms and of animal cells. 

 As soon as the receptors are saturated by these molecular groups, 

 the cells which make use of the former for their nutrition produce 

 them in superabundant quantity. The cells of animals, treated with 

 micro-organisms and their soluble products, or with red blood 

 corpuscles or any other kind of element of animal origin, acquire 

 the property of elaborating more and more of the corresponding 

 receptors, a large proportion of which are expelled into the blood 

 plasma. 



The common point between Ehrlich's theory and the view main- 

 tained in this work consists in the admission of a cellular property 

 which develops more and more in proportion to the treatment of the 

 animal by formed elements of all kinds. As, in acquired immunity 

 against micro-organisms, the fixatives are most frequently found in 

 the body fluids, it must be concluded that, in all these cases, the 

 cells which produce them have become adapted by a kind of 

 education to manufacture increasing quantities of fixatives. But 

 even in those examples of acquired immunity where fixatives are 

 not found in the plasmas, we must accept a modification of the cells 

 which resist the invasion of micro-organisms. These changes in the 

 cellular properties constitute, therefore, the most general, and conse- 

 quently the most important, element in acquired immunity against 

 micro-organisms. 



As already mentioned Ehrlich does not assign any position to 

 the cells which exhibit these modifications. It must, however, be 

 accepted that they belong to the category of phagocytes. Indeed, 

 the phagocytes put themselves into most intimate contact with the 

 micro-organisms and foreign animal cells, and it is in the phagocytic 

 organs that the fixatives are found before they are met with in 

 the blood plasma. It may then be concluded that, in acquired 

 immunity against micro-organisms, the phagocytes become adapted 

 to elaborate the fixatives in large quantities, of which a portion is 

 excreted into the body fluids, as has been shown in many examples of 

 such immunity. 



The progressive adaptation of the phagocytes in intracellular 

 digestion can be demonstrated by the fact that in an immunised 



