Fortress, Fibrile Antigen (Widal) UK

Bacterial suspension which carry antigen will agglutinate on exposure to antibodies to Salmonellaorganisms. Patients’ suffering from enteric fever would possess antibodies in their sera which can react and agglutinate serial doubling dilutions of killed, coloured Salmonella antigens in a agglutination test.

The main principle of widal test is that if homologous antibody is present in patients serum, it will react with respective antigen in the reagent and gives visible clumping on the test card and agglutination in the tube. The antigens used in the test are "H" and "O" antigens of Salmonella Typhi and "H" antigen of S. Paratyphi. The paratyphoid "O" antigen are not employed as they cross react with typhoid "O" antigen due to the sharing of factor 12. "O" antigen is a somatic antigen and “H” antigen is flagellar antigen.