SOME ASPECTS OF THE IMMUNITY QUESTION. 235 



against zymotic disease had been found to be possible, this 

 had been effected by the communication to an animal of a 

 modified form of the disease against which protection was 



sought ". 



Since natural and acquired immunity are possibly differ- 

 ent in principle and even dependent upon various causes, 

 many conceptions have arisen as to the origin of this con- 

 dition, which subsequent investigation has not verified so 

 as to remove the ideas from the position of hypotheses. 

 The immune condition has been considered to be related to 

 the exhaustion or lack of suitable material on which the 

 micro-organisms of infective disease could thrive. General 

 or local modifications in the metabolic activity of the cells of 

 the organism have also been held to be the cause of immunity. 

 This has found its maximum development in the idea of 

 phagocytism, where not only leucocytes (Metschnikoff, 1883) 

 but the cells of the fixed elements of connective tissues 

 play an important role by the inception and subsequent de- 

 struction by chemical means of specific pathogenic germs. 

 Criticism of the phagocyte theory in Germany led to the 

 discovery of the bactericidal action of blood and serum, 

 while the possession of antitoxic properties by the body 

 fluids of animals rendered artificially immune against 

 tetanus or diphtheria, supported the theory that definite anti- 

 bacterial, or antitoxic substances, are present in the immune 

 organism. The work of Buchner (5) and Hankin (6) falls 

 in this period, and the latter was successful in isolating a 

 bacteria-killing globulin. In his own words: "Immunity, 

 whether natural or acquired, is due to the presence of 

 substances, which are formed by the metabolism of the 

 animals rather than by that of the microbe, and these 

 possess the power of destroying either the microbe against 

 which immunity is possessed, or the products on which 

 their pathogenic action depends" (7). Buchner has re- 

 defined his attitude on this question quite recently, and, 

 while criticising the work of Behring, the real founder of 

 the hypothesis that immunity was due to the antitoxic 

 property of blood towards the toxines of tetanus and 

 diphtheria, maintains that neither in vitro nor within the 



