Salmonella enterica serovar Typhi capsular polysaccharide antigen (Vi antigen)

The **Salmonella enterica serovar Typhi capsular polysaccharide antigen** (Vi antigen) is a surface-expressed, high molecular weight, O-acetylated polysaccharide composed of repeating α-1,4-linked N-acetylgalactosaminuronic acid residues[1][9]. It forms a capsule around S. Typhi, distinguishing it from closely related non-typhoidal serovars and serving as a major virulence factor in the pathogenesis of typhoid fever[1][7]. The Vi antigen promotes **immune evasion** by inhibiting phagocytosis by neutrophils while facilitating uptake by macrophages through specific host receptors such as DC-SIGN[3][5]. This dual function aids in the intracellular survival and dissemination of S. Typhi. The Vi antigen is the principal component of several licensed typhoid vaccines, including purified polysaccharide and newer conjugate vaccine forms, which operate by inducing protective antibody responses[4][8]. Measurement of anti-Vi antibody titers serves as a biomarker of immunity. Vi-based vaccines are generally safe, with infrequent and mild adverse effects, but unconjugated forms are less effective in young children, a limitation addressed by the development of conjugate vaccines[8][4].

Other names

Vi capsular polysaccharideVi antigen capsuleVi polysaccharideCapsular polysaccharide ViS. Typhi Vi antigen

Induction of antibody-mediated immunity (vaccines induce anti-Vi antibodies that neutralize or promote clearance of S. Typhi)[2][4][6]; Prevention of S. Typhi infection by blocking the function of capsular polysaccharide[4][8]