At the end of the 19th century, Paul Ehrlich in his side-chain theory coined the term antibody. In immunology, an antigen represented as Ag is a molecular structure or molecule that is present outside the pathogen. The immune response is normally triggered in the body by the presence of antigens. The abbreviation for Ag stands for antibody generator. Each of the antibodies produced in the immune system matches with the antigen when cells come in contact with the antigen. The antibody matches with the antigen in order to bind to it due to adaptation in an antigen-binding fragment of the antibody. In most situations, the adapted antibody can react with the specific antigen.
The antigen definition biology is as follows, a substance that is capable of producing an immune response by activating the lymphocytes. Proteins, peptides, and polysaccharides are considered antigens, lipids, and nucleic acids, when combined with polysaccharides and proteins, become antigens.
The antigens have the ability to originate from the body or they can be developed from the external environment as well. The best example for antigens in the immunogenic form is vaccines. These vaccines are induced intentionally to the recipient towards the memory function of the adaptive immune system. Example: Vaccines provided for the seasonal flu.
The antigenic is a molecule that is capable of binding an antibody or antigen receptor of T cells. The antigenic stimulation study is used for the evaluation of immunodeficiency, in order to determine the functional capabilities of the peripheral blood mononuclear cells to respond to the specific stimuli.
Let us see what types of antigens are there: antigens can be classified based on the source.
Exogenous Antigens: The antigens that are entered into the body from the outer environment are called exogenous antigens. Exogenous antigens are taken into antigen-presenting cells and formed as fragments by the process called phagocytosis or endocytosis. By the use of class II histocompatibility molecules present in the surface, the fragments are then presented to T helper cells. Some of the antigens are started as exogenous antigens and converted into endogenous antigens.
Endogenous Antigens: Due to viral or bacterial infection or by normal cells due to normal cell metabolism, endogenous antigens are formed. The self-reactive T cells or cytotoxic cells are used for presenting the self-proteins. These include heterologous, autologous, and homologous antigens. In an autoimmune system sometimes the antigens are part of the host itself.
Autoantigens: It is a self-protein or protein complex that is recognized by patients who are suffering from some of the specific autoimmune diseases. The self-proteins under normal conditions should not be the target of the immune system. In the case of autoimmune diseases, the T-cells are not deleted instead they attack their target.
Neoantigens: The antigens that are absent from the normal human genome are the neoantigens. By a method called MANA-SRM, which is developed by a diagnostic company the neoantigens are directly detected and qualified. The viral antigens and tumor antigens are included in the neoantigens.
The uses of antigens are as follows:
Locating cellular proteins
Immunization is the process where a person is capable of resistance to infectious diseases. To understand the immunization process additional attempts are made:
The poor immunogenic antigens are transferred to the high immunogenic antigens to form a strong immunogen. The therapeutic compounds that produce allergic reactions are converted into tolerogenic compounds by attaching to the respective compound of the tolerogen. Such types of manipulations that are made for the antigens proves the importance of the preparation of vaccines.
The suppressor T cells may attenuate or prevent the immune response, these attempts are made to eliminate the suppressor cells.