130 IMMUNITY, NATURAL AND ACQUIRED 



(4.) Humoral theory — developed by Behring — so far as 

 active immunity is concerned it may be held as proved that the 

 production of immunity is accompanied by changes in the blood 

 serum, i.e., by the development of anti-microbic or anti-toxic 

 substances. No doubt, however, such substances are produced, 

 not simply by chemical changes in the body fluids, but are 

 products of cellular action brought about by the presence of the 

 bacteria or their toxins. What cells are these ? Metchnikoff 

 says leucocytes, but when a horse is immunised against diphtheria 

 its serum possesses a certain degree of anti-toxic power, so does 

 the serum derived from subsequent bleedings, although fresh 

 leucocytes have been generated after each bleeding. Hence, fresh 

 anti-toxin must have been produced after each bleeding withou 

 the introduction of fresh toxin. Anti-toxin, or the immunity 

 conferring substance, must therefore be a direct product of the 

 tissue cells by virtue of acquired secretory changes, and its 

 action must also be directly cellular. 



I must apologize for this sketchy survey of an intricate subject 

 — a subject which is constantly presenting new phases with 

 increased research, but one which has a peculiar fascination for the 

 thoughtful student as well as for the practical man. 



