The antigen-antibody reaction is widely used in laboratory diagnostics, including immunohaematology. It is a reversible chemical reaction:
antigen + antibody ⇄ antigen - antibody complex
The forces joining the antigen-antibody complex are not strong covalent bonds but weaker bonds, appropriately named “weak interactions”1.
Types of weak interactions
According to quantum mechanics, all chemical bonds are based on electrostatic forces. A list of types of weak bonds is shown in table I, together with their strength (energy). Van der Waals forces are the weakest, but they are able to attract all kinds of molecules. Hydrogen or ion-dipole bonds require oppositely charged atoms. Hydrogen bonds are very important in aqueous solutions because water easily forms strong hydrogen bonds. Actually, “hydrophobic” bonds are simply the result of the exclusion from water molecules of other molecules that cannot participate in good hydrogen bonds. These molecules are called “non-polar” because their atoms do not form electric dipoles. At 37 °C, water molecules have an average kinetic energies higher than the weakest bonds. Moreover, the kinetic energy is not distributed uniformly. Therefore, even the strongest single weak interaction has an ephemeral life of a fraction of a second at physiological temperatures. For a stable binding, several weak bonds must be present contemporaneously and this requires steric complementarity between the molecules1.