Which of the following flagellar antigens is most commonly associated with Salmonella typhi and is used in the Widal test for diagnosis of enteric fever?

Explanation

## Salmonella typhi Antigenic Structure **Key Point:** The H antigen (flagellar antigen) of *Salmonella typhi* is the primary target of antibodies in the Widal test and is essential for motility and serological identification. ### Antigenic Components of S. typhi | Antigen | Nature | Role in Diagnosis | Clinical Significance | |---------|--------|-------------------|----------------------| | **O antigen** | Somatic (LPS core) | Early response; less specific | Present in all Salmonella; shared with other species | | **H antigen** | Flagellar protein | Main Widal test target; phase variation | Highly specific; used for serotyping | | **Vi antigen** | Capsular polysaccharide | Virulence factor; vaccine target | Masks O antigen; found in ~90% of wild-type strains | | **K antigen** | ~~Not applicable to Salmonella~~ | — | This term is used for *E. coli* capsules, not Salmonella | **High-Yield:** The Widal test detects: - Anti-O antibodies (early, less specific) - Anti-H antibodies (specific, diagnostic gold standard in endemic areas) - Anti-Vi antibodies (indicates chronic carrier state) **Clinical Pearl:** H antigen undergoes phase variation (reversible switching between two flagellar types, e.g., d and e,n,x in S. typhi), which is a virulence mechanism allowing immune evasion. **Mnemonic:** **O-H-Vi** = **O**uter (somatic), **H**agella (motility), **V**irulence (capsule).