Hantigenen

Hantigenen, in bacterial immunology often referred to as H antigens, are flagellar surface proteins that function as antigens recognized by the host immune system. They are the basis of serotyping for motile bacteria, especially species in the family Enterobacteriaceae such as Salmonella and Escherichia coli. The H antigen is encoded by flagellin genes (for example fliC; in many Salmonella serovars also fljB). Flagellin proteins form the filament of the flagellum and present specific epitopes that elicit antibody responses. In Salmonella, expression of different H antigens can switch by phase variation, controlled by an invertible promoter and the Hin recombinase. This means a single strain may alternate between two H antigens during infection or culture, complicating serotyping.

Hantigenen are independent from the O antigen (lipopolysaccharide) and K antigen (capsule) components, which together with H antigens define a serovar. The detection and typing of H antigens traditionally use serological assays with antisera; increasingly, molecular methods such as sequencing of flagellin genes or PCR-based typing are used for rapid discrimination.

Because H antigen variation can occur within a lineage, it contributes to epidemiological tracing, outbreak investigation, and informs vaccine design considerations, though vaccines often target multiple antigens. See also O antigens, serotyping, flagellin, Salmonella, Escherichia coli.