IX 



AGGLUTININS 



THE WIDAL PHENOMENON AND OTHER AGGLUTINATION 



REACTIONS 



Agglutinins are antibodies belonging to the 

 second order of Ehrlich (see Chapter III). They 

 are formed in the blood-serum as a result of infection 

 with specific bacteria. Agglutinins have the power 

 of causing the specific bacteria, in response to which 

 they have been called forth, to lose their motility and 

 to be drawn together in clumps. Agglutinins are 

 probably not very important factors in immunity, but 

 are of great value for diagnostic pm'poses, as their ac- 

 tion can be studied in vitro. Thus blood-serum in 

 various dilutions, when brought in contact with a cul- 

 ture of the particular organism that has infected the 

 patient, will after a time cause loss of motility and 

 clumping of the bacteria in the culture. The principal 

 infections in which agglutination tests have proved 

 most serviceable in diagnosis are those caused by ty- 

 phoid, paratyphoid, dysentery, and colon bacilli. 

 These tests are of great value, sometimes in differen- 

 tiating closely related organisms. Agglutinins be- 

 long to what are known as " group-reaction " anti- 

 bodies, that is to say, they act upon closely allied 



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