Antigenic structures may be shared among naturally occurring polymers, including proteins and polysaccharides. Proteins are polymers of amino acids. Cross-reactions between proteins are due to similarities in their overall shape rather than their individual amino acid components. Cross-reactions have been demonstrated among proteins with similar evolutionary development and structure, such as serum albumins or immunoglobulins. Polysaccharides are polymers of monosaccharides. In contrast to proteins, antigenic specificities may be conferred by mono-, di-, and trisaccharides. Since there are about 150 known naturally occurring monosaccharides, it is not unexpected that cross-reactions are demonstrable between polysaccharides from widely divergent sources.