Immunology — MCQs

Question 1: Which is the most immunogenic antigen of Salmonella Typhi?

• A. O antigen
• B. H antigen (Correct Answer)
• C. Vi antigen
• D. Somatic antigen

Explanation: **Explanation:** The immunogenicity of an antigen is determined by its chemical complexity and size. In *Salmonella Typhi*, the **H (Flagellar) antigen** is the most immunogenic because it is composed of proteins (flagellin). Proteins are more potent triggers of the immune system compared to polysaccharides, leading to a robust antibody response. This is why H-agglutinins appear earlier and reach higher titers than O-agglutinins during a *Salmonella* infection. **Analysis of Options:** * **A & D. O Antigen (Somatic Antigen):** These are the same entity. The O antigen is a lipopolysaccharide (LPS) located on the outer membrane. While it is important for serogrouping, polysaccharides are generally less immunogenic than proteins. O-antibodies appear later and disappear sooner than H-antibodies. * **C. Vi Antigen:** This is a surface polysaccharide capsular antigen (Virulence antigen). It is poorly immunogenic and primarily functions by masking the O antigen from antibodies. Its main clinical utility is in identifying chronic carriers and for use in certain vaccines (e.g., Typhim VI). **High-Yield Clinical Pearls for NEET-PG:** * **Widal Test:** Measures antibodies against O and H antigens. A titer of **>1:160 for O** and **>1:160 for H** is usually considered significant in endemic areas. * **Sequence of Appearance:** In Enteric fever, O antibodies appear first (around the end of the 1st week), but H antibodies reach higher peaks and persist longer. * **Carrier State:** Persistent high titers of **Vi antibodies** (1:10 or more) suggest a chronic carrier state, as the bacteria continue to harbor the capsule in the gallbladder or urinary tract.