O Antigen, Bacterial

The lipopolysaccharide-protein somatic Antigens, usually from Gram-Negative Bacteria, important in the serological Classification of enteric bacilli. The O-specific chains determine the specificity of the O Antigens of a given serotype. O Antigens are the immunodominant part of the lipopolysaccharide molecule in the intact bacterial Cell. (From Singleton & Sainsbury, Dictionary of Microbiology and Molecular Biology, 2d ed)