AGGLUTINATION AND PRECIPITATION 153 



at which agglutination occurs, the greater is the specificity of 

 the reaction. As has been seen, the dilution must be great enough 

 in every case to escape the activity of the normal and the common 

 group agglutinins. Serum from an animal that has been immun- 

 ized against a specific organism may be used in the recognition 

 of that organism. Typhoid serum in this way can be used in the 

 detection of the typhoid bacillus. Such a test also enables one to 

 differentiate between closely related forms, as the varieties of the 

 dysentery and of the paratyphoid bacilli. 



Macroscopic Widal, or Agglutination Test. A series of small 

 test-tubes is prepared, each tube containing a definite amount of a 

 suspension of the organism. To these are added varying quantities 

 of serum, making dilutions of 1 : 10, 1 : 50, 1 : 100, 1 : 200, and higher. 

 A positive reaction is indicated by the appearance of small flocculi 

 of bacteria, which soon settle to the bottom, leaving the super- 

 natant liquid clear. The reaction may not be complete for several 

 hours, and the tubes should be allowed to stand for twenty-four 

 hours before making final observations. Check tubes should 

 always be kept as controls, in which the liquid should remain 

 uniformly turbid. Advantage is taken by certain manufacturers 

 of the fact that dead bacteria, as well as the living, may be 

 agglutinated. They make " agglutinometers " containing all 

 the apparatus and materials for making a complete test. The 

 bacterial suspension supplied will keep for a long period. 



Significance of Agglutinins in Immunity. Agglutination takes 

 place in the body as well as without. Clumps of typhoid bacilli 

 may be found in various capillaries in cases of typhoid. It is 

 uncertain what significance is to be attached to the phenomenon. 

 It can scarcely be of advantage, except possibly that it prevents 

 to some degree the distribution of the bacteria through the blood. 



Hemagglutinins. Certain bacteria, and certain toxic materials 

 from plants and animals, contain substances which agglutinate 

 red blood-cells. Such agglutinins are termed hemagglutinins. 

 When this agglutination occurs in the blood, it results in the 

 formation of emboli of the red blood-cells. These emboli are of 

 considerable significance in some diseases. Hemagglutinins may 

 also be formed by the injection of the red blood-cells of one species 

 into another. 



