Clinical significance of Vi antigen of S typhi is -

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.