O antigen

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(microbiology) A repetitive glycan polymer attached to the core oligosaccharide of the LPS layer of certain bacteria; a somatic antigen that comprises the outermost domain of the lipopolysaccharide

(immunology) A major blood group antigen indicating the presence of H antigen and the lack of additional antigens, particularly N-Acetylgalactosamine (found in A antigen) and galactose (in B antigen)


In microbiology, O antigen renders additional protection from the antimicrobial action of certain antibiotics. Without it, the LPS becomes hydrophobic and therefore is more susceptible to the action of hydrophobic antibiotics. 1

In immunology, blood types are determined by the presence of blood-group antigens. In humas, there are three types of blood-group antigens, namely H antigen, A antigen, and B antigen. They differ basically on structural component.


  • (microbiology) O polysaccharide
  • (microbiology) O side-chain


See also:


  • 1Tsujimoto H et al. (2003). "Diffusion of macrolide antibiotics through the outer membrane of Moraxella catarrhalis". Journal of Infection and Chemotherapy 74 (4): 1045–1055. doi:10.1007/s101569900025