An unheated suspension of Salmonella Typhi typically produces agglutination of Vi antisera. After heating the same suspension, agglutination will occur in which grouping sera?

• A. A
• B. B
• C. D
• D. C1

Answer: (C)

The main idea here is that the Vi capsule in Salmonella Typhi can mask the underlying somatic (O) antigens, and heating the suspension can inactivate that capsule. When the suspension is unheated, the Vi antigen drives agglutination with Vi antisera. heating eliminates the Vi capsule, so the reaction with Vi antisera disappears and the exposed O antigens become accessible to grouping sera that target those somatic antigens.

After heating, agglutination is seen with the grouping sera that recognizes the particular O antigen now exposed, which corresponds to the group associated with that serovar’s somatic antigen. In this case, that corresponds to the group designated by the C grouping sera.

So, heating shifts the reaction from Vi antisera toward the grouping sera that detects the exposed O antigen (the C group). The other groupings wouldn’t typically react in this heated state because their target epitopes are not revealed or present after capsule destruction.