THE HUMORAL THEORY. 491 



farther back, namely to the phagocytes. The phenomena 

 of phagocytosis so admirably demonstrated by Metchnikoff 

 may be regarded as the results of immunity, but cannot be 

 accepted as its cause. 



5. 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 development 

 of antimicrobic or antitoxic substances. No doubt, how- 

 ever, 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 certain amount of 

 'evidence, which it is unnecessary to detail, has been brought 

 forward by Metchnikoff, Bordet and others to show that 

 both bactericidal substances of normal serum and specific 

 substances of antimicrobic sera are derived from leuco- 

 cytes. In this way the theory of phagocytosis has under- 

 gone modification. Similar evidence with regard to the 

 origin of antitoxic substances is wanting. The whole 

 question, however, is still an open one, and there is 

 evidence that the formation of these bodies is not restricted 

 to one class of cells but takes place in various tissues. 



