[263] CHAPTER IX 



THE MECHANISM OF ACQUIRED IMMUNITY AGAINST 

 MICRO-ORGANISMS 



Cytases and fixatives. Only the latter are augmented in the immunised organism. 

 Properties of the fixatives. Difference between them and the agglutinative 

 substances. The part played by the latter in acquired immunity. Protective 

 property of the fluids of the immunised organism. Stimulant action of the body 

 fluids. The protective power of serum cannot serve as a measure of acquired 

 immunity. Examples of acquired immunity in which the serums exhibit no 

 protective power. Phagocytosis in acquired immunity. Negative chemiotaxis 

 of leucocytes. Theory of attenuation of micro-organisms by the fluids of im- 

 munised animals. Refutation of this theory. Phagocytosis acts without requiring 

 any previous neutralisation of the toxins. The origin of the fixative and pro- 

 tective properties of the body fluids. The relation between these properties and 

 phagocytosis. The side-chain theory of Ehrlich and the theory of phagocytes. 



WHILST, in natural immunity against micro-organisms, humoral 

 phenomena play no prominent part, in acquired immunity these 

 phenomena assume a much greater importance. The bactericidal 

 power of the fluids of the body is, in natural immunity, reduced to a 

 mere trace, for it has been demonstrated that the power of normal 

 serums to destroy bacteria corresponds to no natural phenomenon of 

 the living organism, but is dependent upon the presence of cytases 

 which have escaped from the phagocytes at the time of the formation 

 of the clot in vitro and separation of the serum. The presence of the 

 fixative, that other important element in immunity, has been demon- 

 strated in the normal fluids only in rare cases and in small quantity. 

 The agglutinative property of these fluids has likewise shown itself 

 to be little developed and without any importance in natural 

 immunity. 



In acquired immunity against micro-organisms, on the other 

 hand, we find that the bactericidal and agglutinative powers of the 



