<prompt> Clinical significance of Vi antigen of S typhi is -Helps in diagnosisHighly immunogenicMost important antigen for widal testAntibody against Vi-antigen is used for diagnosis of carrier  <response>: Ans. is d i.e., Antibody against Vi-antigen is used for diagnosis of carrier Antigen structures of salmonellao Three important antigenic structures are:-1 Flagellar antigen H It is a heat labile protein and present on flagella. The H antigen is strongly immunogenic and induce antibody formation rapidly and in high titres following infection or immunization. H agglutinins persist longer than O agglutinins.2 Somatic antigen O It is an integral part of cell wall. It is a phospholipid- protein-polysaccheride complex and is identical to endotoxin. It is less immunogenic than H antigen, antibody production is slow and lower in titre.3 Vi antigen It is a heat labile polysaccharide which envelops O antigen and prevents its agglutination by O antiserum It acts as a virulence factor by inhibiting phagocytosis and complement mediated killing. It is poorly immunogenic, only low titres of antibody are produced following infection. Detection of Vi antibody is not helpful for the diagnosis of cases and hence ve Vi antigen is not employed in widal test. Vi antibody disappears early in convalescence, its persistence indicates the development of the carrier state. Total absence of Vi antibody in a proven case of typhoid fever indicates poor prognosis. 