The O antigen is a component of the outer membrane of certain bacteria, particularly Gram-negative bacteria. It is a polysaccharide chain that forms part of the lipopolysaccharide (LPS) structure, which is crucial for the bacteria's structural integrity and protection against environmental factors. The O antigen varies among different bacterial strains, contributing to their serological classification. In the context of bacterial infections, the O antigen plays a significant role in the immune response. It can trigger the production of antibodies in the host, which helps the immune system recognize and combat the bacteria. Understanding the O antigen is important for developing vaccines and treatments against bacterial diseases.