CHAPTER XV 

 AGGLUTINATION AND PRECIPITATION 



(Antibodies of Ehrlich's Second Order) 



GRUBER, in 1896, discovered that the blood of animals immun- 

 ized against Bacillus typhosus, and the blood of patients having the 

 disease, when added to a liquid culture of the organism, caused 

 the bacteria to cease moving and to clump together. This 

 phenomenon has been named agglutination. Later it was found 

 that the use of protein substances as antigens caused the pro- 

 duction in the body of. substances which, when mixed with the 

 protein in solution, would form a precipitate. This is known as 

 the precipitation phenomenon. The antibody responsible for 

 agglutination is called an agglutinin; for precipitation, a predpitin. 



Differentiation of Precipitation and Agglutination. The dis- 

 tinction between agglutination and precipitation may be stated as 

 follows: Agglutination occurs when the antigen is in suspension 

 in the form of individual cells or finely divided particles. Pre- 

 cipitation occurs when the antigen is a colloid in solution. 



Agglutination. Agglutinins may be grouped into two classes, 

 normal and immune. A normal agglutinin is one present in the 

 body without any infection or systematic immunization. An 

 immune agglutinin is one that is developed as a result of the pres- 

 ence of an organism or its products in the body. There is no 

 reason to suppose that the normal and immune agglutinins differ 

 from each other in any essential particular. It is possible that all 

 normal agglutinins are in reality produced as a result of an unde- 

 tected infection or to the presence of the so-called group agglu- 

 tinins to be considered later. 



The agglutination reaction is said to be specific; that is, the 

 agglutinin will agglutinate, in general, only the homologous 

 organism. The term homologous is used to indicate the relation- 



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