222 A MANUAL OF BACTERIOLOGY 



hanging-drop preparation ; the organisms first lose their 

 motility and soon become aggregated into large masses 

 or clumps. Macroscopically, the reaction may be followed 

 in a narrow test-tube into which the mixture of culture 

 and serum has been introduced ; after some hours the 

 micro-organisms become aggregated into masses so large 

 as to form visible flocculi. which in time tend to subside 

 to the bottom of the tube. The substances which bring 

 about this agglutination are known as agglutinins. 

 Agglutinins are present in small amount in normal serum ; 

 for instance, most normal human sera up to a dilution of 

 1 in 5 or 1 in 10 will agglutinate the typhoid bacillus and 

 still more powerfully the glanders bacillus. Agglutinin 

 is anti-body, formed by the action of antigen derived 

 from the bacterial cell, and agglutination is brought about 

 by the action of the agglutinin on the antigen. 



The agglutinin of an agglutinating serum usually 

 agglutinates only the homologous organism, and in the 

 process is taken up and absorbed by the organism, and 

 more or less completely removed from the serum. This 

 interaction is to some extent quantitative, and a given 

 amount of culture absorbs only a certain maximum amount 

 of agglutinin. The fact that an agglutinating serum 

 for the most part agglutinates only its homologous 

 organism renders agglutination a valuable test for ascer- 

 taining the nature of a micro-organism, and it is commonly 

 available for bacteria, yeasts and protozoa. Moreover, 

 in the process of a natural infection, agglutinins are 

 similarly formed and accumulate in the serum, so that 

 agglutination may be employed as a very useful test of the 

 nature of an infection. 



The fact that agglutinin is absorbed by the homologous 

 organism may be made use of as a valuable extension of 

 the agglutination test, and it is even more delicate than 

 the latter. Thus, an agglutinating serum may agglutinate 



