Difference Between O and H Antigen

The key difference between O and H antigen is that O antigen is the outermost portion of the bacteria’s surface covering while H antigen is the slender threadlike structure that is part of the flagella.

Antigens are molecular recognition sites present in many bacteria, fungi, viruses, dust particles and other cellular and non-cellular particles which could be recognized by the host immune system. Generally, most antigens are present on the cell surface. Structurally, antigens can be proteins, amino acids, lipids, glycolipids or glycoproteins or nucleic acid markers. These molecules possess a specific ability to bring about an immune response in the host. This immune response is brought about by triggering the production of antibodies as a corresponding result.

Antigens are generally used in serotype identification system for the bacteria. Salmonella is a bacterial genus that has many serotypes. Serotypes are groups of microorganisms within the same species. These serotypes can be distinguished using the combination of O antigen and H antigen. O antigen is an LPS side chain while H antigen is a part of the flagella.

1. Overview and Key Difference
2. What is O Antigen
3. What is H Antigen
4. Similarities Between O and H Antigen
5. Side by Side Comparison – O vs H Antigen in Tabular Form
6. Summary

What is O Antigen?

O antigen is the outermost portion of the bacteria’s surface covering. It is a somatic antigen. In fact, it is a polysaccharide which is a part of the cell wall lipopolysaccharide. Based on the composition of the oligosaccharides of lipopolysaccharide, O antigens can be determined in each serotype.

Figure 01: O Antigen

O antigens are heat stable and alcohol resistant. But O antigens are formaldehyde labile. Among the bacterial serotypes, O antigen specificity is determined by the sugar sequence of the polysaccharide chains.

What is H Antigen?

H antigen is a part of flagella. It is the slender threadlike structure of the flagella. H antigen is composed of flagellin protein. Hence, it is a proteinaceous antigen. Unlike O antigen, H antigen is not a part of the cell wall or a somatic antigen. It is a flagellar antigen.

Figure 02: H Antigen

H antigens are heat-labile and sensitive to alcohol. But they are formaldehyde stable. H antigens persist for a long time, and they are highly immunogenic.

What are the Similarities Between O and H Antigen?

Scientists use a combination of O and H antigens in order to determine serotypes.
Widal test measures antibodies against H and O antigens.

What is the Difference Between O and H Antigen?

O antigens are major components of the surface lipopolysaccharide (LPS) of bacteria while H antigens are slender threadlike part of the flagella of bacteria. So, this is the key difference between O and H antigen. Besides, the O antigen is a polysaccharide while H antigen is a protein.

Moreover, O antigens are composed of polysaccharides; hence, they are heat stable. But, H antigens are made up of protein; hence they are heat-labile. Furthermore, O antigens are alcohol-resistant while H antigens are alcohol sensitive.

The below infographic of the difference between O and H antigen shows more comparisons between both.

Summary – O vs H Antigen

O antigen and H antigen are two types of antigens used to determine the different serotypes of bacterial species. O antigen is a lipopolysaccharide found on the cell wall. H antigen is a proteinaceous antigen which is a part of the flagella. Therefore, O antigen is a somatic antigen while H antigen is a flagellar antigen. Thus, this summarizes the difference between O and H antigen.

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Image Courtesy:

1. “LPS” By Mike Jones – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Kauffmann-White-Schema Antigen” By Bacteria-.svg: JrPolFile:Enterobacteriaceae Antigen.svg Matthias M. – FileEnterobacteriaceae Antigen.svg (CC BY-SA 3.0) via Commons Wikimedia