In antibody binding, an antigen is material bearing a recognized epitope. An antigen need not be a protein; polysaccharides can also carry epitopes. One antigen may present several different epitopes, so recognizing one feature does not imply recognizing every part of the target. [ref]
The interaction of the antibody molecule with specific antigen. Immunobiology, 5th edition, §3-8: small recognized regions of protein and polysaccharide antigens
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