ABSTRACT
The reaction between antigens (Ags) and antibodies (Abs) involves complementary binding sites on the Ab and on the Ag molecules. The sites on the Ag molecule that combine with the binding site of an Ab are known as epitopes. The same way that the binding site is determined by different segments on the variable regions of heavy and light chains that come in close proximity due to the folding of those regions, the epitopes are also formed by discontinuous segments of an Ag molecule. Crystallographic studies have defined protein epitopes as large areas, usually involving 15 to 22 aminoacids located on several surface loops. Some subsets of aminoacids within the epitope are likely to contribute most of the binding energy with the Ab, whereas the surrounding residues provide structural complementary that may play a stabilizing role when Ags and Abs interact. The significance of critical aminoacids in an epitope is illustrated by the fact that Abs can distinguish immunoglobulin allotypes structurally determined by one or two aminoacid substitutions in the constant regions of heavy or light chains (see Chapter 7).
