What is the antigen in the novel coronavirus antigen detection kit?

               The interaction between antigen and antibody is the basis of immunochemistry. As an effective research tool, it is necessary to know how the antibody binds to the corresponding antigen. The characteristics of the interaction between antigen and antibody mainly include the following three points:

1. Specificity of antigen antibody binding

        Antigenic determinants interact with the antigen binding sites of antibodies in a non covalent manner,        and they must be in close contact at the empty space to generate sufficient binding force. The complementary structure between molecules determines the specificity of antigen antibody binding. Among them, there are four kinds of interactions or affinities between antigen and antibody molecules: hydrogen bond, ionic bond, hydrophobic interaction and van der Waals interaction.

         Minor changes in antigen structure will affect the strength of antibody antigen interaction. The loss of a hydrogen bond at the interface reduces the strength of the interaction. The interaction is the balance between the attractive force and the repulsive force of the contact surface. The change of amino acid residues at the binding site can also change the intensity of antigen antibody interaction.

2. Reversibility of antigen antibody binding

       Affinity was used to measure the binding strength of epitope and antibody. The noncovalent binding of antigen and antibody is reversible (Fig. 3), and the intensity of the interaction can be described as an equilibrium reaction. If [AB] is the mass concentration of the free antibody binding site, [Ag] is the mass concentration of the free antigen binding site, and [abag] is the mass concentration of the antigen antibody complex, the affinity calculation formula is [abag] / [AB] [Ag].

        The time to reach equilibrium depends on the diffusion rate, and the antibody with high affinity can bind the antigen in a shorter time than the antibody with low affinity. The application of high affinity antibody in immunochemistry is effective, which is related to its high activity and the stability of the complex. For example, the dissociation half-life of a high affinity antibody binding to a small molecule protein antigen is 30 minutes or more, while the dissociation time of a low affinity antibody is only a few minutes or less.

        The affinity of antibody antigen interaction is different, and the affinity constant of antibody antigen interaction is affected by temperature, pH and solvent. When these conditions are changed, it can be found that when the reaction equilibrium is reached, the number of antigen antibody complexes increases or decreases, which promotes the reaction to fully bind or dissociate the bound antigen. According to this characteristic, the antigen or antibody can be separated and purified by affinity chromatography.

3. Relationship between antigen amount and antibody reaction

         Affinity refers to the overall stability of antigen antibody complexes. The overall intensity of antibody antigen interaction is controlled by three factors: the intrinsic affinity of antibody to epitope, the valence of antibody and antigen, and the three-dimensional structure of reaction components. The antibody has multivalent binding ability, in which IgG and most IgA are bivalent and IgM is decavalent. The antigen may be multivalent or monovalent. Multivalent interactions can stabilize immune complexes

             If a multivalent antigen is mixed with a specific antibody in a test tube,   an immune complex can be formed. When the concentration of antigen and antibody is appropriate, that is, in the equivalent region,   antigen and antibody molecules are widely cross-linked through non covalent bonds to form large immune complexes. If the antigen is excessive,      due to the reversibility of the binding of antigen and antibody,    the free antigen will replace the bound antigen, dissociate 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 bound antibody, and the antigen bound to the antibody will be dissociated and will not form a large immune complex

         The three characteristics of antigen antibody interaction, i.e. the relationship between specificity, reversibility and antigen antibody reaction amount, are briefly described above. Antigen antibody reaction is the basis of immunochemistry.   For example,     humoral immunity is based on the binding of antigen and antibody molecules,       which can bind on the cell surface or in free form. Through the combination of antigen and antibody, the physiological effects such as regulation, precipitation, agglutination, phagocytosis and cell lysis are triggered to form a complete set of humoral immune functions of the body.

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