THE AGGLUTINATION REACTION 



265 



the corresponding antisera also will contain certain agglutinins in 

 common which can unite with both types of organisms, and thus 

 give rise to agglutination in the lower dilutions. As the more 

 specific receptors, however, predominate over those that are com- 

 mon to both types, the corresponding agglutinins will also be more 

 abundant in the antisera, so that the type of infection can be 

 established from the higher dilutions in which a serum will cause 

 agglutination of a given organism. 



FIG. 15 



Positive agglutination reaction. 



A material advance in the practical applicability of the Widal reac- 

 tion was achieved when it was discovered that it is not necessary 

 to work with living cultures of the typhoid bacillus, but that dead 

 organisms will answer just as well, providing that the strain was 

 readily agglutinable before being killed. To this end it is convenient 

 to prepare a bouillon culture in an Erlenmeyer flask, to incubate 

 for twenty-four hours at 37 to 40 C., and then to add 40 per cent, 

 formalin solution (i. e., the concentrated solution of the pharmacopeia) 

 to the extent of 1 per cent. After standing for two to five days in 

 the incubator, the emulsion is centrifugalized, the bacilli are washed 

 with two changes of sterile normal salt solution and diluted to the 

 original volume, when the fluid emulsion may be preserved in sterile 



