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an agglutination reaction test used to diagnose typhoid fever, proposed in 1896 by the French physician F. Widal (1862-1929). The reaction is based on the ability of antibodies (agglutinins) formed in the body during a disease and persisting long after recovery to cause typhoid microorganisms to clump together. If agglutination takes place when a culture of the causative agent is added to human serum, the reaction is considered positive. The Widal reaction is run several times in the diagnosis of typhoid fever, and the results are considered dynamically and in relation to the anamnesis.