298 IMMUNOLOGICAL METHODS OF DIAGNOSIS 



receptors in common, 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 dilu- 

 tions. As the more specific receptors, however, predominate over 

 those that are common to both types, the corresponding agglutinins 

 will also be more abundant in the antisera, so that the type of infec- 

 tion can be established from the higher dilutions in which a serum 

 will cause agglutination of a given organism. 



A material advance in the practical applicability of the Widal 

 reaction was achieved when it was discovered that it is not neces- 



Positive agglutination reaction. 



sary 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 con- 

 venient 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 

 Pharmacopoeia) 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 



