Antigen (Ag) refers to all substances that induce an immune response in the body. That is, it can specifically recognize and bind to the antigen receptor (TCR/BCR) on the surface of T/B lymphocytes, and activate T/B cells. A substance that proliferates and differentiates to produce an immune response product (sensitized lymphocytes or antibodies) and which specifically binds to the corresponding product in vitro and in vivo. Therefore, antigenic substances possess two important characteristics: immunogenicity and immunoreactivity. Immunogenicity refers to the ability of an antigen to induce a specific immune response in the body to produce antibodies and/or sensitize lymphocytes; immunoreactivity refers to the ability to react with the corresponding immune effector (antibodies or sensitized lymphocytes) in vitro and in vivo

The basic properties of antigens are heterogeneous, macromolecular and specific:

(1) Heterogeneous refers to an antigenic substance that enters the body tissues and is different from the components of the body tissue cells. Antigens generally refer to foreign substances entering the body, such as bacteria, viruses, pollen, etc.; antigens can be substances from different species, for example, some proteins in horse serum become rabbit antigenic substances when they are into rabbits. The allogenic substance can also become an antigen, such as blood type, transplant immunity, etc. Some isolated components from the body can also become antigens, such as eye crystal proteins, sperm cells, thyroglobulin, etc. Under normal circumstances, it is fixed at a certain part of the body and is isolated from the antibody-producing cells, so it does not cause autologous antibody production. However, when traumatized or infected, these components enter the bloodstream and can cause the production of autologous antibodies, just like foreign bodies. These substances that are antigenic to themselves are called autoantigens, and the antibodies produced are called autoantibodies. When autoantibodies react with autoantigens, autoimmune diseases such as allergic ophthalmia and thyroiditis happened. Other tissues proteins of the body can also become autoantigens due to the denaturation of physicochemical and biological factors such as ionizing radiation, burns, certain chemicals and certain microorganisms, causing autoimmune diseases such as lupus erythematosus and leukopenia disease, chronic hepatitis, etc.

(2) Macromolecular means that the substance constituting the antigen is usually a macromolecular substance having a relative molecular mass of more than 10,000, and the larger the molecular weight, the stronger the antigenicity. Most proteins are good antigens. Why are antigenic substances all macromolecules? This is because macromolecular substances can stay in the body for a long time, and have enough time to contact with immune cells (mainly macrophages, T lymphocytes and B lymphocytes), causing immune cells to respond. If the foreign substance is a small molecule, it will be quickly excreted by the body, and there is no chance to contact the immune cells. If the macromolecular protein is hydrolyzed into a small molecule, the antigenicity is lost.

(3) Specificity means that an antigen can only specifically bind to a corresponding antibody or effector T cell. The specificity of an antigen is determined by specific chemical groups on the surface of the molecule, and these chemical groups are called antigenic determinants. The antigen activates lymphocytes to cause an immune response by binding the antigenic determinant to the antigen receptor of the corresponding lymphocyte. In other words, the antigen recognition receptor on the surface of lymphocytes distinguishes between "self" and "dissident" by recognizing the antigenic determinant. The antigen also binds to corresponding antibody through the antigenic determinant, eliciting immune response. Therefore, antigenic determinants are the material basis for the immune response and immune response.

…to be continued in part two.

Reference

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