148 IMMUNITY. THEORIES OF IMMUNITY 



attributed to it, yet, throughout the discussion of immunity the im- 

 portance of the phagocyte itself is emphasized. This theory of Metch- 

 nikoff is treated more fully in the chapter on Phagocytosis. 



SIDE-CHAIN THEORY 



The humoral theory of immunity, which would ascribe the power to 

 resist infection to the body-fluids, may be said to have had its origin in 

 1896, when Fodor discovered that the blood of the rabbit will kill an- 

 thrax bacilli in the test-tube, independent of cells and phagocytosis. 

 Later Buchner adopted this theory, and sought to explain the bacteri- 

 cidal action of blood-serum as dependent upon a special constituent 

 which* he called alexin. 



With the discovery, in 1890, of antitoxins by von Behring and Kit- 

 asato, the theory received fresh support, and while an effort was made 

 to demonstrate that antitoxins were of paramount importance in ac- 

 quired immunity, evidence soon accumulated to show that this anti- 

 toxic power is operative only in a few diseases, chiefly in diphtheria and 

 tetanus. 



Fresh support to the "humoral" as against the " cellular" explana- 

 tion of immunity was given by Pfeiffer in 1894, with the discovery that 

 cholera vibrios introduced into the peritoneal cavity of a guinea-pig 

 previously immunized against cholera became transformed into granules, 

 and ultimately passed into complete solution (bacteriolysis), apparently 

 without the aid of cells. Bordet then showed that this phenomenon 

 was due to two distinct substances one, the "sensitizing substance," 

 which is specific and exists only in the immune serum, acting only on the 

 bacteria against which the animal was immunized, and the other a non- 

 specific substance, found in the fresh serum of practically all animals, 

 and to which he gave the name "alexin," and which was later renamed 

 by Ehrlich and called "complement." 



Of the various theories offered in explanation of these observations, 

 the suggestive, fascinating, though highly hypothetic theory of Ehrlich, 

 known as the side-chain theory, has been most widely accepted and 

 adopted to explain new discoveries as they were made. The theory 

 has, indeed, aided investigators in making new discoveries. Nevertheless 

 the contention of Bordet, that its too ready acceptance without sufficient 

 convincing proof has retarded investigation, should not be ignored. 



The basis of this theory, as originally proposed, bore no relation to 

 the subject of immunity, but was advanced in 1885 to explain the pro- 

 cesses of nutrition. 



