156 IMMUNITY. THEORIES OF IMMUNITY 



principles not being contradictory; they may, rather, be regarded as 

 one set of phenomena viewed from different aspects. 



Since its original announcement, Metchnikoff has, on different oc- 

 casions, enlarged upon his theory to meet certain discoveries, made 

 chiefly by adherents of Ehrlich's theory, showing the presence of sub- 

 stances in the blood-serum and other body-fluids that are potent in the 

 processes of immunity independent of cells. Metchnikoff claims, 

 however, that these antibodies are derived from the group of cells 

 classified as phagocytes, and thus would preserve the primary impor- 

 tance of his theory. Ehrlich, on the other hand, while not denying that 

 these cells may be a source of their formation, points out that they are 

 not necessarily the sole or supreme source, but may be formed by the 

 general body-cells or by special groups of cells possessing a selective 

 affinity for the pathogenic agent. 



The theory of Ehrlich is essentially a chemical one, and maintains 

 that the union of food or pathologic material with cells is a chemical 

 union; his views, therefore, possess that degree of definiteness necessary 

 to constitute a plausible chemical theory. The theory of Metchnikoff 

 would explain processes of nutrition and immunity as largely founded 

 on a physical basis, and is therefore, necessarily more general, being 

 largely biologic and vitalistic. 



The two theories differ in two more or less hypothetic points: (1) 

 In the manner by which material enters into relation with cells, and 

 (2) the relative importance of certain cells in the formation of anti- 

 bodies. Otherwise both are intimately related, in that phagocytosis is 

 unimportant if removed from the influence of antibodies in the body- 

 fluids, and these same antibodies, although probably formed accord- 

 ing to Ehrfich's theory, are derived in part from Metchnikoff's phago- 

 cytes. 



Phagocytosis, whether by leukocytes, endothelial cells, or by newly 

 developed connective-tissue cells, is very common, and is obviously a 

 most important factor in the destruction of pathogenic bacteria and in 

 the cure of infectious disease. In virulent infections, however, phagocy- 

 tosis may not be apparent ; the leukocytes are not attracted, and those 

 in the vicinity undergo dissolution. Later in these infections, however, 

 phagocytosis may become apparent, due, according to Metchnikoff, to 

 the " adaptation" of the cells to the products of the invading micro- 

 organism, whereby the weak or negative chemotaxis is converted into an 

 active positive chemotaxis with vigorous digestion. This, however, is 

 not primarily due to increased digestive capacity of the phagocytes, 



