IMMUNITY 195 



The other, thermostable, only exists (with rare exceptions) in 

 treated immunized animals : in the example given above it 

 makes the cholera vibrio susceptible to the action of the 

 alexine or complement, or it may be said to .fix this to the 

 bacteria, or, finally, it may be regarded as forming the connect- 

 ing link between the alexine or complement and the vibrios. 

 It has been called by Bordet the ' sensibilisatrice.' ^ 



The idea of two substances is not a theory, as Bordet 

 remarks with great justice ; there is nothing hypothetical in 

 it. It is simply putting in words the facts observed, in 

 particular those of re-activation. 



Haemolysis, agglutination, precipitation, in a word, all the 

 reactions in which antibodies play a part, proceed in the same 

 fashion and Bordet's discoveries have thus a general applica- 

 tion. 



A fundamental experiment performed by EhrUch and 

 Morgenroth completes Bordet's observations. The immune 

 body becomes fixed on the antigens (bacteria or blood 

 corpuscles) without producing in them any visible modification, 

 so that eventually the fluid containing it is completely robbed 

 of immune body; this fixation prepares the way for the 

 destruction of the corpuscles or the bacteria, but this latter does 

 not occur till the complement is added. On the other hand, 

 the complement does not fix itself to the antigen when alone, 

 but only by the intermediation of the immune body. 



This collaboration of two substances or ferments in one 

 complex physiological process is not the sole example in biology. 

 The digestive ferments of the pancreas, amylase, saponase, 

 and especially trypsin, fail to exert their full activity without the 

 collaboration of the enterokinase, a ferment secreted in the 

 juice of the small intestine not by the cells and glands of the 

 mucous membrane itself, but by the lymphoid tissue, which is 

 composed of white corpuscles : the importance of this fact will 

 be seen (Pawlow; Chepowalnikoff ; Delezenne). 



' The alexine is called by Ehrlich the complement or complementary 

 substance ; the sensibilisatrice is often called the immunizing substance ; the 

 intermediate substance, immune-body or amboceptor. 



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