The pathogenic and immunogenic activities of<i>Salmonella typhi</i>in relation to its antigenic constituents

1. The most virulent strains of Salmonella typhi are those that develop both the Vi and O antigens in maximum quantities. 2. The Vi antigen is not a particularly toxic substance; it is less toxic for the mouse than the O antigen. 3. The main biological function of the Vi antigen is to protect the O antigen against the action of the natural or immune O antibody, thereby protecting the bacterial cell against phagocytosis and the bactericidal action of the serum. 4. The relative virulence for mice of freshly isolated strains of Salmonella typhi reflects their virulence for man.