CHAPTER XXIII 



THE MECHANISM OF AXTIGEX- 

 AXTIBODY REACTIONS 



WHEN an antigen and its corresponding antibody 

 are brought into contact they react and the 

 reaction manifests itself in a manner depending 

 on the nature of the antigen and the conditions prevailing 

 at the time of reaction. If the antigen is a soluble sub- 

 stance like a toxin or a serum protein precipitation may 

 occur ; if the antigen is carried by a bacterial cell or a 

 red blood corpuscle agglutination may result, or the cell 

 m.ay be rendered sensitive to lysis or phagocytosis, which 

 Vv'ill take place if complement or leucocytes are also 

 present. From the majority of evidence available it 

 appears that antigen-antibody reactions occur in two 

 stages. The first stage consists in the direct combination 

 of the two reagents, the reaction being specific ; this is 

 followed by a non-specific stage, in the sense that one of 

 several phenomena, such as flocculation, agglutination, 

 complement fixation or lysis, may occur depending on 

 the physical conditions operating at the time. Thus if 

 bacteria are treated with the corresponding anti-serum 

 in water instead of saline as the medium, no agglutination 

 occurs, in spite of the fact that the two reagents have 

 com^bined, as can be shown by separating the organisms 

 by centrifugalisation, leaving a supernatant fluid devoid 

 of anti])ody. If the sensitised deposit is suspended in 

 saline agglutination immediiitely takes place. 



In the first stage of the reaction between cintigeji and 

 antibody the antigen can be replaced by the corresponding 



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