The interaction of antigen and antibody is the basis of immunochemical technology. As an effective research tool, antibodies need to know how the antibody binds to the corresponding antigen. The characteristics of the interaction between antigen and antibody mainly have the following three points:
The antigenic determinant and the antigen-binding site of the antibody interact in a non-covalent manner. The two must be in close contact in space to produce sufficient binding force. Small changes in antigen structure can affect the strength of antibody-antigen interactions. The loss of a single hydrogen bond at the contact surface can reduce the strength of the interaction. The interaction is carried out between the balance of attraction and repulsion of the contact surface. Changes in the amino acid residues of the binding site can also change the strength of the antigen-antibody interaction.
Affinity is used to determine the strength of epitope and antibody binding. The time to reach equilibrium depends on the diffusion rate. High-affinity antibodies can bind to the antigen in a shorter time than low-affinity antibodies. High-affinity antibodies are better used in immunochemical techniques because of their higher activity and the stability of the complex. For example, the dissociation half-life of a high-affinity antibody binding to a small protein antigen is 30 minutes or longer, while the dissociation time of a low-affinity antibody is only a few minutes or less. The affinity of the antibody-antigen interaction is different. The affinity constant of the antibody-antigen interaction is affected by temperature, pH and solvent. Changes in these conditions can find that the number of antigen-antibody complexes increases or decreases when the reaction equilibrium is reached, prompting the reaction progresses towards complete binding or dissociates the bound antigen. Based on this property, affinity chromatography can be used to separate and purify the antigen or antibody.
Avidity refers to the overall stability of the antigen-antibody complex. The overall strength of the antibody-antigen interaction is controlled by three factors: the intrinsic affinity of the antibody to the epitope, the valence of the antibody and the antigen, and the three-dimensional structure of the components involved in the reaction. If the multivalent antigen is mixed with specific antibodies in a test tube, immune complexes can be formed. When the concentration of antigen and antibody is appropriate, that is, the zone of equivalence, the antigen and antibody molecules are extensively cross-linked through non-covalent bonds to form large immune complexes. If there is an excess of antigen, due to the reversibility of antigen and antibody binding, the free antigen will replace the bound antigen, dissociating the antibody that has bound the antigen, and cannot form a large immune complex; similarly, if the antibody is excessive, the free antibody will replace the already bound antibody, dissociate the already bound antibody antigen, and cannot form a large immune complex.
The above briefly described the three characteristics of antigen-antibody interactions, namely the specificity, reversibility and medium-quantity of the antigen-antibody reaction. Antigen-antibody reaction is the basis of immunochemistry. For example, the basis of humoral immunity is the combination of antigen and antibody molecules. This combination can be bound to the cell surface or in a free form. The combination of antigen and antibody then triggers physiological effects, such as regulation, precipitation, agglutination, phagocytosis, cytolytic effect, etc., forming a complete set of humoral immune functions.