MICEOBES— INFECTION— IMMUNITY 463 



sometimes the chemical substance exercises a repeUing 

 effect on the phagocyte, and this is known as negative 

 chemiotaxis. 



According to this theory natural immunity is due to 

 active chemiotaxis, the invading material being engulphed 

 and digested ; when however this fails to occur through the 

 existence of negative chemiotaxis, infection results. 



The action of a preventive serum in the body is due to 

 increased positive chemiotaxis, the result of the co-operation 

 of two ferments in the serum. One of these, the 'Jixateur,' 

 is contained in the liquid blood and acts on the bacteria for 

 the phagocyte to deal with, while the other ferment, known 

 as cytasis, is contained within the bodies of the phagocytes 

 from which it only escapes after the coagulation of the 

 blood (bactericidal serum). 



Bacterial immunity accordingly represents nothing else 

 but an example of intra-cellular digestion. 



Natural and acquired immunity against toxins are 

 according to Metchnikoff due to some vital influence of 

 certain cells, which prevent the attack of the toxins on 

 susceptible cells. 



In the case of acquired immunity probably macrophages 

 form the principal source of antitoxins, and these in the 

 living body act in a similar way to the ' fixateur ' in the 

 case of bacteria. 



The essential difference between the two theories of 

 infection and immunity, of which we have given only a 

 mere outline, is that, while Erlich considers the destruction 

 as taking place in the liquid blood, Metchnikoff regards it 

 as occurring in the phagocytic cells. 



Metchnikoff's theory is essentially vital, Ehrlich's essen- 

 tially chemical. 



Methods of Producing Immunity.— These will be dealt 

 with in sufficient detail in considering the various 

 infectious diseases, but we may here conveniently classify 

 the methods which are of practical value in dealing with 

 animal diseases. 



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