THE HUMORAL THEORY. 451 



the facts of immunity. The insufficiency of the theory, how- 

 ever, was at once apparent when the method of immunising 

 against a toxine was discovered ; and the facts discovered 

 later, with regard to the action of antimicrobic sera, showed 

 that the cellular ingestion of bacteria was not the most 

 important factor in immunity against the living organisms. 

 The theory as originally propounded is, accordingly, no 

 longer tenable, and even if it were consistent with facts it 

 only removes the property of immunity a step farther back, 

 namely, to the phagocytes. The phenomena of phago- 

 cytosis so admirably demonstrated by Metchnikoff may be 

 regarded as the results of immunity, but cannot be accepted 

 as its cause. 



4. The Humoral Theory. This theory, which ascribed 

 immunity to changes in the serum and other fluids of the 

 body, was chiefly developed by Behring and by others of 

 the German school. It may be said to have originated with 

 the discovery of bactericidal power possessed by normal 

 blood serum ; and the earlier work consisted in an attempt 

 to explain natural and acquired immunity by supposing 

 changes to take place in this bactericidal power. It is, 

 however, unnecessary to state the various phases through 

 which the theory has passed, as these are now chiefly 

 of historic interest. So far as active immunity is 

 concerned, it may be held as proved that certainly the 

 appearance of immunity is accompanied by changes in 

 the serum, as described above, that is, by the develop- 

 ment of antimicrobic or antitoxic substances. No 

 doubt, however, such substances are not produced simply 

 by chemical changes in the body fluids, but are products 

 of cellular action brought about by the presence of 

 the bacteria or their toxines. The question remains as 

 to which cells chiefly generate such substances. A con- 

 siderable amount of evidence, which it is unnecessary to 

 detail, has been brought forward by Metchnikoff, Bordet 

 and others to show that both bactericidal substances and 

 the indirectly bactericidal substances of antimicrobic sera, 

 are derived from leucocytes. In this way the theory of 



