O antigen and H antigen are different in that O is the bacteria’s outermost layer, while H is the flagella’s threadlike structure.

Antigens can be defined as molecular recognition points found in bacteria, viruses, fungi, dust particles, and other cellular or non-cellular particles that could be recognized and treated by the immune system. Most antigens can be found on the cell’s surface. Antigens can be structurally proteins, amino acids or glycolipids. These molecules have the ability to trigger an immune response within the host. The immune response is triggered by the production of antibodies.

In the identification of bacteria’s serotypes, antigens are used. Salmonella, a bacterial genus, has many serotypes. Serotypes can be described as groups of microorganisms belonging to the same species. These serotypes are distinguished by the combination of O and H antigen. O antigen can be considered an LPS sidechain, while H antigen can be described as a part the flagella.

What is O Antigen?

O antigen refers to the outermost part of bacteria’s skin. It is a somatic antigen. It is actually a polysaccharide that is part of the cell wall lipopolysaccharide. O antigens can also be identified in each serotype based on the composition of the lipopolysaccharides.

O antigens can withstand heat and alcohol. But O antigens are formaldehyde labile. O antigen specificity among bacterial serotypes is determined by the sugar sequence in the polysaccharide chain.

What is H Antigen?

Flagella contains H antigen. It is the threadlike, slender structure of flagella. Flagellin protein is the main component of H antigen. It is therefore a proteinaceous antibiotic. H antigen is different from O antigen. It is not part of the cell walls or a somatic. It is a flagellar antibody.

H antigens can be sensitive to alcohol and are heat-labile. They are however formaldehyde-stable. They are immunogenic and can persist for a long period of time.

What is the Difference Between O and H Antigen?

O antigens make up the major components of the bacteria’s surface lipopolysaccharide. H antigens, on the other hand, are the threadlike parts of the bacteria flagella. This is the main difference between O antigen and H antigen. The O antigen, on the other hand, is a polysaccharide. H antigen is protein.

O antigens also contain polysaccharides, which makes them heat stable. However, H antigens contain protein and are therefore heat-labile. O antigens can withstand alcohol, while H antigens can withstand alcohol.

Character	O Antigen	H Antigen
Referred to as	Somatic Antigen or Boivin antigen	Flagellar antigen
Determination	Based on oligosaccharides associated with lipopolysaccharide.	Based on flagellar proteins.
Cell wall	Part of the cell wall lipopolysaccharide (LPS).	Not a part of the cell wall.
Composition	Polysaccharide.	Proteinaceous (Flagellin).
Heat sensitivity	Somatic antigens are heat stable.	Flagellar antigens are heat-labile.
Alcohol sensitivity	Resistance to alcohol	Sensitive to alcohol
Formaldehyde sensitivity	Formaldehyde labile	Formaldehyde stable
Extraction	Trichloro-acetic acid is used for extraction of O antigens.Since the property was first shown by Boivin, O antigen alternatively referred to as boivin antigen.	Formaldehyde is used for extraction of H antigens.
Immunogenicity	Less immunogenic	Highly immunogenic
Antibody levels	Produces antibody formation with low titres.	Induces antibody formation with high titres.
Antibody formation	Rapid and Early	Rapid and Sustained
Lifespan	Antibody levels fall off quickly.	Persists for longer periods.
Antibody indicates	O antibody appears early, disappears early: indicates recent infection.	H antibody appears late, disappears late: lndicates convalescent stage.
Type of agglutination reaction shown	Produces compact, chalky and granular clumps.	Produces cottony, fluffy precipitates.
Reaction time	Agglutination takes place slowly	Agglutination takes place rapidly.
Optimum temperature for reaction	Optimum temperature for agglutination is 55’°C.	Optimum temperature for agglutination is 37’°C.
Reaction observed with	Round bottom Felix tube are used to see agglutination.	Conical bottom Dreyer’s tube is used to see agglutination.
Role as virulence factor	The most important virulence factor responsible for endotoxic activity; it protects the bacteria from phagocytosis and bactericidal effect of complement.	Makes the bacteria motile, hence contributing to their virulence.
Existence in phases	No phases	Flagellar antigens exist in two alternative phases- Phase I and II.Most o f them are biphasic except S. Typhi which is monophasic.
Widal test	In Widal test, O antigen of Salmonella Typhi is used.	In WidaI test, H antigens of S.Typhi, S.Paratyphi A and B are used.
Use in classification	Serogrouping of salmonellae is based on the O antigen.	Serogroups are differentiated into serotypes based on H antigen.