Antigens stimulate the immune system to produce antibodies. Antigens are usually specific proteins on the surface of bacteria or viruses. In autoimmune diseases, the recognition of antigens is impaired and the body’s own tissues are fought as foreign antigens.
What are antigens?
Antigens are the substances to which the lymphocytes of the immune system form antibodies. Lymphocyte receptors and antibodies can bind specifically to antigens, stimulating antibody production and protective immune responses. To be distinguished from antigenicity is immunogenicity. Antigenicity refers to the ability to bind to a specific antibody. Immunogenicity, on the other hand, refers to the ability to induce a specific immune response. Medicine distinguishes between full antigens and half antigens. Full antigens independently trigger the formation of certain antibodies. Semi-antigens or haptens are not capable of this. They require a so-called carrier, i.e., a protein body that allows them to become a full antigen.
Anatomy and structure
Usually, antigens are proteins or otherwise complex molecules. Less commonly, they correspond to carbohydrates or lipids. Smaller molecules do not usually elicit immune responses on their own and therefore cannot be called antigens. An antigen is usually composed of antigenic substructures. These substructures are also called determinants or epitopes. They bind either to B-cell receptors, to T-cell receptors or directly to antibodies. B-cell receptors and antibodies recognize and bind the antigens on the surface of invaded foreign bodies. These antigens have a three-dimensional structure, which is one of the most important recognition features for B-cell receptors and antibodies. T-cell receptors recognize antigens from denatured peptide sequences of about ten amino acids. These amino acids are taken up by antigen-presenting cells. Together with MHC molecules, they are presented at the surface.
Function and roles
The human immune system has hereditarily encoded receptors for certain substances. Thus, it can recognize many foreign substances as a danger to the body and fight them by immune reactions. However, the organism does not have hereditarily encoded receptors against every type of substance. In this respect, antigen recognition by the lymphocytes protects the organism against foreign substances for which there are no hereditarily encoded receptors. The binding of a lymphocyte to foreign substances triggers an adaptive immune response. Antigens thus initiate the formation of different antibodies. These antibodies bind with the epitope present and contain the threats. Thus, it is the recognition of exogenous antigens that enables the immune system to target invaders such as viruses without harming the body’s own cells. While hereditarily encoded receptors of the immune system can assess certain substances as dangerous from the outset, the immune response in the context of antigen recognition is, so to speak, linked to a learning process of the immune system. Once the body has been in contact with the antigen of a certain bacterium or virus, specific antibodies are present for this substance, which help to combat the supposed threat the next time contact is made with the antigen. Incidentally, the human body also contains antigens. However, the immune system develops a tolerance to these endogenous antigens and therefore recognizes them as harmless. The glycoprotein structures on the cell surface of human tissue are different for each person. Tolerance can therefore develop specifically and differentially to one’s own antibodies. The body tissue of another person is then still recognized and fought against as an antigen foreign to the body. This makes transplantation more difficult, for example. The immune system of a transplant recipient often recognizes the transplanted tissue as an exogenous antigen, against which it develops specific antibodies. For this reason, the tolerance of the tissue must always be taken into account during transplantations. Meanwhile, transplant patients are also given immunosuppressants that block the process described.
Diseases
Allergies are an overreaction to certain antigens. In the context of allergic diseases, the immune system considers foreign antigens to be more dangerous than they actually are.Impaired antigen recognition is also present in autoimmune diseases. In these diseases, an immune response is initiated against the body’s own antigens. Normally, the immune system is tolerant to the body’s own substances. In autoimmune diseases, however, this tolerance breaks down. To date, the exact cause of autoimmune diseases is unclear. The sequestration theory assumes that many endogenous antigens were not in the immediate vicinity of these immune cells during the development of tolerance. These endogenous antigens can then not be recognized as endogenous if there is direct contact at some point. If such a direct contact between the immune cells and the endogenous antigens occurs, for example due to an injury, then they are attacked as endogenous antigens. Other theories assume a change in the body’s own antigens in the context of certain viral infections or drugs as the cause for the attack of endogenous substances. Whichever theory is correct: in any case, faulty antigen recognition is the basis of autoimmune diseases. A well-known example of such a disease is the inflammatory disease multiple sclerosis, in which the patient’s own immune system attacks tissue of the central nervous system, triggering destructive inflammation in the brain or spinal cord. However, the reverse case also harbors dangers. For example, the body can also develop a tolerance to foreign antigens. The immune system then no longer attacks these tolerated antigens, exposing the organism to great danger.
