8.1 Characteristics of biologic antigens

1) One of the most important characters of antigen is to bind specifically to an antibody.

2) Almost all the antigens are identified by specific antibodies but very few have the ability to stimulate the antibodies. Sometimes in order to provoke an immune response, immunologists adjoin several copies of small molecules called hapten to a protein prior to immunization and the protein to which it is attached is known as carrier .

3) Foreign antigens are usually much bigger than the region where actual binding occurs between the antigen and the antibody and this region is known as antigen binding region. An antibody prefers to bind to this small region of the antigen known as epitope . Epitopes are hence also called as antigenic determinants .

4) Random structure on the antigenic molecule that is identified by the antibody as an antigenic binding site forms the epitope of that antigen.

5) Different epitopes are so organized on a single protein molecule that their spacing may affect the binding of antibody molecules in various ways.

8.2 Chemistry of antigen binding

The interaction of an antigen antibody is a reversible binding process that requires several non-covalent interactions like hydrogen bonds, electrostatic forces and hydrophobic interactions. Affinity and Avidity between the antigen antibodies also play a major role in their interaction. The potency of the reaction between a specific antigenic determinant and its single combining site on the antibody determines its affinity . The overall potential of binding of an antigen with many antigenic determinants to its multivalent antibody determines its avidity . Normally antigen-antibody binding site on antibodies are more or less flat and hence spacious so that they can attach large complexes or structures.