Historical sketch on Immunity 543 



of raising them to the dignity of theories, and of late years they have 

 not been upheld. 



It cannot be denied that not one of the humoral theories has been 

 able to retain its position or to stand against the numerous facts that 

 have been accumulated during recent years. 



This extraordinary discrepancy between the bactericidal power of 

 the body fluids and immunity is explained by the circumstance that 

 the microbicidal substances exist in the living animal within phagocytes 

 and only escape from them when these cells have been injured. The 

 fact, so well demonstrated by Gengou, that the blood plasma is without 

 any bactericidal power has given the final blow to the microbicidal 

 theory of the body fluids and it can no longer be maintained. 



The humoral theories, based on the antitoxic and protective power 

 of the body fluids, can claim only a very restricted application. These 

 properties are met with in acquired immunity only, and even there 

 are not constant. Many cases of acquired immunity against micro- [567] 

 organisms are unaccompanied by any antitoxic power, and in several 

 examples of this immunity the body fluids do not exhibit any pro- 

 tective power. 



There is only one constant element in immunity, whether innate i 

 or acquired, and that is phagocytosis. The extension and importance j 

 of this factor can no longer be denied. 



It is clearly proved that phagocytes are susceptible cells which 

 react against morbific agents, whether organised or not. These cells 

 ingest micro-organisms and absorb soluble substances. They seize 

 microbes whilst these are still living and capable of exercising their 

 noxious effect and bring them under the action of their cellular 

 contents, which are capable of killing and digesting the micro- 

 organisms or of inhibiting their pathogenic action. Phagocytes act 

 because they possess vital properties and a faculty of exerting a 

 fermentative action on morbific agents. The mechanism of this action 

 is not yet definitely settled, and we can foresee that for future 

 researches there will be a vast and fertile field to be reached by 

 pursuing this path. 



The present phase of the question of immunity constitutes one 

 stage only in the development of biological science and one which 

 is capable of many improvements. 



