3.3.3. Antigen-Antibody Interaction
Lock and Key Type Binding Epitopes. Foreign molecules that generate antibodies are called antigens. Antigen molecules each have a set of antigenic determinants also called epitopes (Fig. 3.12). The epitopes on one antigen (Ag1 in Fig. 3.12) are usually different from those on another (Ag2 in Fig. 3.12)). Some antigens (Ag3 in Fig. 3.12) have repeated epitopes. Epitopes are molecular shapes recognized by the antibodies and cells of the adaptive immune system. Each cell recognizes one epitope rather than the whole antigen. Even simple microorganisms have many different antigens.
Nature of Binding Forces. There are several types of intermolecular attractive forces binding antigen to antibody. These forces require the close approach of the interacting groups. Hydrogen bonding results from the formation of hydrogen bridges between appropriate atoms; electrostatic forces are due to the attraction of oppositely charged groups located on two protein side chains. Van der Waals bonds are generated by the interaction between electron clouds (here represented as induced oscillating dipoles) and hydrophobic bonds (which may contribute up to half the total strength of the antigen-antibody bond) rely upon the association of non-polar, hydrophobic groups so that contact with water molecules is minimized. The distance of separation between the interacting groups which produces optimum binding varies for the different types of bond (see Fig. 3.13).