O-antigen is a component of the outer membrane of certain bacteria, particularly Gram-negative bacteria. It is a polysaccharide chain that helps to define the serotype of the bacteria, which is important for identifying different strains. The O-antigen plays a crucial role in the immune response, as it can help bacteria evade detection by the host's immune system. The structure of the O-antigen varies among different bacterial species, contributing to their diversity. It is often used in laboratory settings for serotyping, which is a method to classify bacteria based on their surface antigens. Understanding O-antigens is essential for developing vaccines and treatments against bacterial infections.