Most immunogenic in typhoid is: March 2011

Ans. B: H antigen The H antigen is strongly immunogenic and induces antibody formation rapidly and in high titres following infection or immunization Somatic (O) or Cell Wall Antigens They are heat stable and alcohol resistant. Surface (Envelope) Antigens Commonly observed in other genera of enteric bacteria (e.g., Escherichia coli and Klebsiella), may be found in some Salmonella serovars. Surface antigens in Salmonella may mask O antigens, and the bacteria will not be agglutinated with O antisera. One specific surface antigen is well known: the Vi antigen. Vi antigen The Vi antigen occurs in only three Salmonella serovars (out of about 2,200): Typhi, Paratyphi C, and Dublin. Strains of these three serovars may or may not have the Vi antigen. Flagellar (H) Antigens Flagellar antigens are heat-labile proteins. Strongly immunogenic; induces antibody formation rapidly and in high titres following infection or immunization Mixing salmonella cells with flagella-specific antisera gives a characteristic pattern of agglutination (bacteria are loosely attached to each other by their flagella and can be dissociated by shaking). Also, antiflagellar antibodies can immobilize bacteria with corresponding H antigens. A few Salmonella enterica serovars (e.g., Enteritidis, Typhi) produce flagella which always have the same antigenic specificity. Such an H antigen is then called monophasic. Most Salmonella serovars, however, can alternatively produce flagella with two different H antigenic specificities. The H antigen is then called diphasic.