Agglutination with '0' antigen of S. typhi is inhibited by -

Ans. is 'a' i.e., Vi antigen . Many strains of S. typhi fail to agglutinate with 0 antiserum when freshly isolated. This is due to presence of a surface polysaccharide antigen enveloping the '0' antigen. Antigenic structures of Salmonella : 1. Flagellar antigen 'I-1' Present on the flagella Heat labile protein Strongly immunogenic and induces antibody formation rapidly and in high titres following infection or immunisation. 2. Somatic antigen '0' Is an integral pa of the cell wall A phospholipid-protein-polysaccharide complex Is identical with endotoxin. Less immunogenic than H antigen, antibody production is slow and lower in titre. Salmonellae are classified into a number of groups based on the presence of characteristic '0' antigen on the bacterial surface. 3. Vi antigen Envelops the '0' antigen and prevents its agglutination with 0 antiserum. Is a polysaccharide Is heat labile Tends to be lost on serial cultures Acts as a virulence factor by inhibiting ? - Phagocytosis - Resisting complement activation - Prevent bacterial lysis by the alternative pathway - Prevent peroxidase mediated killing 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.