80 APPLIED IMMUNOLOGY 



bacilli may be employed, and have the advantage of 

 keeping in good condition for some time. With ordi- 

 nary laboratory facilities, however, the microscopic 

 method is preferable. 

 Occurrence and Significance of the Widal Reaction 



The Widal reaction is usually not seen in typhoid 

 fever until the second week of the disease, as some 

 time is required for the production of the specific ag- 

 glutinins in the patient's serum, upon which the test 

 depends. The reaction usually persists for several 

 months after recovery from typhoid fever, and per- 

 sons have been known to react positively for years 

 after the infection. Artificial immunization with ty- 

 phoid vaccine also gives rise to a positive reaction 

 which persists for some time. These facts should be 

 remembered in weighing the results of the test per- 

 formed in suspected cases of the disease. The diag- 

 nosis of typhoid fever should not rest upon the agglu- 

 tination reaction alone. Other symptoms must be con- 

 sidered along with the laboratory test. The reaction 

 may fail to appear in a small percentage of cases of 

 undoubted typhoid fever, and may be positive occa- 

 sionally in other conditions, particularly in jaundice. 

 Probably the range of error on either side does not ex- 

 ceed 3 per cent., so that in the Widal reaction we have 

 one of the most important cardinal signs of typhoid 

 fever. 



