MICEOBES— INFECTION— IMMUNITY 461 



These two-grouped ' receptors,' now called ' amboceptors ' 

 (Fig. 187), after becoming detached from the cell, link up 

 with the bacteria in the following way : — viz. the cytophile 

 group joins on to the bacteria, while the complementophile 

 group unites with the complemental substance which is con- 

 stantly present in the serum of normal animals, and acts as 

 a ferment for the destruction of the bacterium now linked 

 to it. 



In this way the bacterium cannot reach the protoplasm 

 of the cell, and the immunity is known as passive. 



Ehrlich considers active immunity to be due to the 

 co-operation of these two substances, viz. the amboceptors 

 and the complement, and he calls a bactericidal serum 

 ' complex ' which always has these two substances present 

 in it. 



The production of a preventive serum is not only 

 observed after injecting bacteria, but after injecting any 

 cell material derived from animals; for instance, the red 

 corpuscles of species A if injected into species B, produce 

 in B a serum which dissolves the red corpuscles of A. 

 This serum behaves exactly as the anti-bacteric, and it also 

 consists of two substances. Such a serum is termed cyto- 

 toxic, or in the example given above hsemolytic. 



The hyperproduction of amboceptors is according to 

 Ehrlich nothing else but a normal function of the nutrition 

 of the cell, and the immunity it produces is therefore only 

 one of its particular duties. 



Agglutination. — By the injection of bacteria or cells into 

 an animal, an ' anti-body ' is produced which agglutinates 

 the bacteria, so that if added in vitro to a corresponding 

 culture of bacteria, the latter collect in fiocculi and sink 

 to the bottom of the glass. 



This reaction is used for diagnostic purposes, and permits 

 the identification of a species of unknown bacteria by a 

 serum produced by known bacteria ; further, it serves for 

 the diagnosis of disease, the phenomenon being known as 

 agglutination. 



Precipitines are another form of anti-body, for example, 



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