Introduction to Antigen-Antibody Reactions

The interactions between antigens and antibodies are known as antigen–antibody reactions. The reactions are highly specific, and an antigen reacts only with antibodies produced by itself or with closely related antigens. Antibodies recognize molecular shapes (epitopes) on antigens. Generally, the better the fit of the epitope (in terms of geometry and chemical character) to the antibody combining site, the more favorable the interactions that will be formed between the antibody and antigen and the higher the affinity of the antibody for antigen. The affinity of the antibody for the antigen is one of the most important factors in determining antibody efficacy in vivo.

The antigen- antibody interaction is bimolecular irreversible association between antigen and antibody. The association between antigen and antibody includes various non-covalent interactions between epitope (antigenic determinant) and variable region (VH/VL) domain of antibody.

Chemical Bonds Responsible for the Antigen–Antibody Reaction

The interaction between the Ab-binding site and the epitope involves exclusively non-covalent bonds, in a similar manner to that in which proteins bind to their cellular receptors, or enzymes bind to their substrates. The binding is reversible and can be prevented or dissociated by high ionic strength or extreme pH. The following intermolecular forces are involved in Ag–Ab binding:

1. Electrostatic bonds: This result from the attraction between oppositely charged ionic groups of two protein side chains; for example, an ionized amino group (NH4+) on a lysine in the Ab, and an ionized carboxyl group (COO_) on an aspartate residue in the Ag.
2. Hydrogen bonding: When the Ag and Ab are in very close proximity, relatively weak hydrogen bonds can be formed between hydrophilic groups (e.g., OH and C=O, NH and C=O, and NH and OH groups).
3. Hydrophobic interactions: Hydrophobic groups, such as the side chains of valine, leucine, and phenylalanine, tend to associate due to Van der Waals bonding and coalesce in an aqueous environment, excluding water molecules from their surroundings. As a consequence, the distance between them decreases, enhancing the energies of attraction involved. This type of interaction is estimated to contribute up to 50% of the total strength of the Ag–Ab bond.
4. Van der Waals bonds: These forces depend upon interactions between the “electron clouds” that surround the Ag and Ab molecules. The interaction has been compared to that which might exist between alternating dipoles in two molecules, alternating in such a way that, at any given moment, oppositely oriented dipoles will be present in closely apposed areas of the Ag and Ab molecules.

Each of these non-covalent interactions operates over very short distance (generally about 1 Å) so, Ag-Ab interactions depends on very close fit between antigen and antibody.

Strength of Ag-Ab interactions

1. Affinity

• Combined strength of totalnon-covalent interactions between single Ag- binding site of Ab and single epitope is affinity of Ab for that epitope.
• Low affinity Ab: Bind Ag weakly and dissociates readily.
• High affinity Ab: Bind Ag tightly and remain bound longer.

2. Avidity

• Strength of multiple interactions between multivalent Ab and Ag is avidity. Avidity is better measure of binding capacity of antibody than affinity. High avidity can compensate low affinity.

3. Cross reactivity

• Antibody elicited by one Ag can cross react with unrelated Ag if they share identical epitopeor have similar chemical properties.

Types of Ag-Ab reactions

1. Agglutination
2. Precipitation
3. Complement Fixation
4. Enzyme Linked Immunosorbent Assay
5. RadioImmuno Assay
6. Western Blotting

Introduction to Antigen-Antibody Reactions