In the body, with the manifestation of the immune response, antibodies and antigens interact. However, under certain conditions, the latter can cause a state of so-called specific irresponsibility - tolerance. Antibodies and antigens contribute to the formation of immunological memory. Next, consider the second type of substance. In the article we find out what is an antigen.
General information
What is an antigen? Simply put, these are usually foreign compounds. These include nucleic acids, polysaccharides, proteins and their complexes. When modified by chemical modification of natural polymers , "conjugated" substances can be obtained. Such compounds can be formed based on proteins that belong directly to the recipient himself. An autologous substance denatured chemically or physically can also be converted into an antigen.
Definition
Biopolymers or their synthetic analogues, which can cause an immune response, can enter the body. These compounds are called antigens. They contribute to the production of thymic effector cells. Antibodies appearing against the background of an immune reaction begin to specifically interact with antigens or chemical compounds that have a similar structure. If the latter do not provoke a protective response, then they are called haptens. They provoke immunological tolerance. Synthetic polypeptides acting as protein antigens have the ability to induce a protective reaction. However, not necessarily their primary and spatial structure should be similar to that of any particular protein compound. An essential factor in the manifestation of the antigenic properties of these substances is the formation of a stable spatial structure. In this regard, polymers formed from one amino acid (homopolymers) do not have the ability to elicit an immune response. Antigenic abilities appear in polypeptides, the formation of which involves 2 amino acids.
Research questions
What is an antigen? Classical immunology calls such a substance a whole cell of animal or bacterial origin. However, this is incorrect from a chemical point of view. It is said above what an antigen is in essence. This is not a cell in which a large number of nucleic acids, proteins, polysaccharides are present. Purified human antigens can be used to induce an immune response. At the same time, it will be specific for one or another biopolymer. Considering the purified structure as an individual antigen, any combination of them must be described as a family of individual compounds. This term can be used to refer to a spontaneously aggregating specific biopolymer. An example is some antigens of viruses or bacteria. Thus, the contractile protein of flagella of gram-negative microorganisms of the genus Salmonella, flagellin can be detected both in polymerized and monomeric form. In both cases, this antigen can induce the formation of antibodies, despite the fact that the conditions for this are different. In particular, the phellagelin polymer is thymus-independent, and the monomer is thymus-independent.
Molecular weight bond
It can only be established by comparing substances of the same class. For example, this applies to various proteins with the same tertiary and secondary structures: fibrillar and globular. In such cases, a direct relationship can be established between the ability of the polymer to induce the formation of antibodies and its molecular weight. This pattern, however, is not absolute. Among other things, it depends on other properties of the compound, both chemical and biological.
The degree of manifestation of properties
The severity of the antigenic characteristics of the proteins, acting as the most extensive and significant class, will depend on the degree of remoteness in evolutionary terms of the donor from which the compound is obtained and the recipient to which it is introduced. A comparative analysis will be correct only if the same substances were used in the assessment. For example, if rat and human serum albumin are immunized in mice, then the first response will be more pronounced. If the biopolymer is characterized by increased sensitivity to cleavage, then its properties will be less pronounced than that of a substance exhibiting greater resistance to enzymatic hydrolysis. So, in the case of using synthetic polypeptides or protein conjugates as antigens, the response to that substance containing unnatural D – amino acids will be more pronounced. The decisive role in the manifestation of the immune response is assigned to the recipient genotype.
Determinant Groups
They denote the molecular regions of a biopolymer, its synthetic analogue or conjugated antigen, which are recognized by antigen-binding B-lymphocyte receptors and antibodies. A molecule usually contains several determinant groups that are different in structure. Each of them can be repeated several times. If only one group with a certain structure is present in the molecule of the compound, the formation of antibodies against it will not occur. In the process of increasing identical complexes, the immune response to them will increase. However, this process will go on until a certain point, after which it will decrease and may not be observed at all subsequently. This phenomenon was investigated in the process of using conjugated antigens with different numbers of substituents performing the task of the determinant group. The lack of an immune response to biopolymers with increased epitope density is due to the activation mechanism of B-group lymphocytes.
Cancer-embryonic antigen
It is one of the varieties of proteins of normal tissue, which in healthy people is produced to a small extent by the cells of some organs. CEA in its chemical structure is a combination of carbohydrates and protein. Its purpose in adults is unknown. However, during the period of intrauterine formation, it is synthesized quite intensively by the organs of the digestive system, while performing quite important tasks. They are associated with the stimulation of cell reproduction. Cancer-embryonic antigen is detected in the tissues of the digestive organs, but in a fairly small amount. The name of this tumor marker partly characterizes its biological nature, but for the most part all the same properties that are valuable in a laboratory study. The term "embryonic" has a relationship with physiological tasks during development in the prenatal period, the "antigen" indicates the possibility of identification in biological environments using the immunochemical method of binding. Moreover, it does not exhibit any properties directly in the body. Normally, in a healthy organism, the concentration of CEA is quite low. Against the background of the oncological process, its level increases quite sharply, reaching quite large indicators. In this regard, it is characterized as a tissue marker of oncological pathologies, or tumor marker.
CEA level
Antigen analysis is used in the diagnosis of various malignant neoplasms, mainly cancer of the rectum and colon. The study is carried out in the early stages of pathologies, in the process of monitoring the course of the disease and monitoring the effectiveness of therapeutic measures. Against the background of colon and rectal cancer, the test is characterized by the highest sensitivity. This is what allows it to be used in primary diagnostics. After a successful operation to remove all tumor tissue, the CEA concentration returns to normal after a maximum of two months. Regular tests subsequently allow you to evaluate the patient's condition after receiving treatment. The detection of a high level of CEA allows timely detection of a relapse of the pathology. With a decrease in antigen content during therapy, experts conclude that the therapeutic effect is effective.
Increased CEA concentration: range of pathologies
However, the test is not considered absolutely specific for tumors. An increase in the level of CEA can be observed against the background of various diseases of the internal organs, which have an inflammatory and other nature. In 20-50% of patients with benign pathologies of the pancreas, intestines, lungs and liver, the antigen concentration increases slightly. The same is observed against the background of cirrhosis, chronic hepatitis, ulcerative colitis, cystic fibrosis, emphysema, bronchitis, Crohn's disease, pancreatitis, pneumonia, autoimmune diseases, tuberculosis. In addition, an increase in the level can be caused not by a disease, but, for example, by regular drinking or smoking.
Features of blood transfusion
The main one is the specificity and personality that red blood cell antigens possess. If the recipient and donor biopolymers are incompatible, blood transfusion is strictly prohibited. Otherwise, pathological processes and even the death of the patient are inevitable. In immunogenetics, serological reaction methods are used to test and study erythrocyte antigens . These include, in particular, the reactions of hemolysis, precipitation, agglutination. Erythrocyte genes are represented as complex biopolymer macromolecules. They accumulate on the stroma (shell) and are connected with other molecules of the compounds. Each individual is characterized by an individual chemical composition and its own structure.