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In this article we will discuss about the characteristics of epitopes.
1. An antibody that is specific for an antigen binds non-covalently to a region of the molecule surface known as epitope.
2. Naturally occurring epitopes are relatively small (either amino-acids or sugar residues).
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3. Specific epitope should fit with the specific site present on antibody (antibody-binding site).
4. The site present on antibody called antigen- combining site or paratope is a cave pocket shaped one to match with the epitope having a convex site (Fig. 4.8).
5. Small antigens are mainly mono-epitopic where as large proteins and oligosaccharides can express many different and/or identical repeating multi-epitopes.
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6. The forces responsible for binding include hydrophobic and Vander Waals forces, which are spherical, symmetrical and hydrogen bridges, which are directional and require matching of the reactants. Electrostatic forces might also contribute, but they act at distance.
7. Formation of stable immune complexes normally occur only when the epitope and paratope fit ‘Jigsow Fashion’.
8. Not only the position of on epitope within a large molecule is important in determining its ability to induce an immune response, but the position of each subunit within the epitope may also be important. For e.g. each of the amino acid residues comprising a given accessible epitope might unequally contribute to bind with an antibody paratope. Thus some components of an epitope are more immuno-dominant than others.
9. Any amino acid can contribute to a protein epitope. The residues that are not part of the epitope might not bind to antibody but might influence antigen conformation and affect epitope binding. Substitution of even a single amino acid in an epitope can affect binding of antibody.
