What is an antigen? Antibodies and antigens

In the body, when an immune response develops, 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'll find out what an antigen is.

General information

What is an antigen? Simply put, these are, as a rule, foreign compounds. These include nucleic acids, polysaccharides, proteins and their complexes. When the natural polymers are modified by chemical modification, "conjugated" substances can be obtained. Such compounds can be formed on the basis of proteins that belong directly to the recipient itself. An autologous substance denatured by chemical or physical means can also be converted into an antigen.

Definition

Biopolymers or synthetic analogues capable of eliciting an immune response can enter the body. These compounds are called antigens. They contribute to the development of thymic-type effector cells. Emerging against the background of the immune reaction, antibodies begin to interact in a specific way with antigens or chemical compounds having a similar structure. If the latter do not provoke a protective response, then they are called haptens. It is they that provoke immunological tolerance. The ability to induce a protective reaction has synthetic polypeptides, acting as protein antigens. However, optionally their primary and spatial structure should be similar to that of any particular protein compound. An essential factor in the manifestation of antigenic properties in these substances is the formation of a stable spatial structure. In this regard, polymers formed from one amino acid (homopolymers) do not have the properties 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 an animal or bacterial origin. However, this is incorrect from a chemical point of view. It is said above, what is an antigen in essence. This is not a cell in which there is a large number of nucleic acids, proteins, polysaccharides. Human antigens obtained in purified form can be used to induce an immune reaction. In this case, it will be specific for a particular biopolymer. Considering the purified structure as an individual antigen, any combination thereof should be described as a family of individual compounds. This term can be used to designate spontaneously aggregating a certain biopolymer. Some antigens of viruses or bacteria serve as an example. 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 either case, this antigen can induce the formation of antibodies, although the conditions for this are different. In particular, the felagelin polymer is timusone-independent, and the monomer is thymus-dependent.

Relationship with molecular weight

It can be established only by comparing substances of the same class. For example, this applies to different proteins with the same type of 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.

Degree of manifestation of properties

The expression of the antigenic characteristics of proteins acting as the most extensive and significant class will depend on the degree of remoteness in the evolutionary relationship of the donor from which the compound is obtained and the recipient to whom it is administered. A comparative analysis will be correct only if the same types of substances are used in the assessment. For example, if the rat serum albumin and human immunize mice, the first answer will be more pronounced. If the biopolymer is more susceptible to cleavage, its properties will be less pronounced than that of a substance exhibiting greater resistance to enzymatic hydrolysis. So, in case of using synthetic polypeptides or protein conjugates as antigens, the answer to that substance in which non-natural D-amino acids are present is more pronounced. The decisive role in the manifestation of the immune response is assigned to the recipient's genotype.

Determinant groups

They denote the molecular parts of the biopolymer, its synthetic analogue or conjugated antigen, which are recognized by antigen-binding B-lymphocyte receptors and antibodies. In a molecule, there are usually several determinant groups, different in their structure. Each of them can be repeated several times. If there is only one group with a certain structure in the molecule of the compound, antibodies will not form against it. In the process of increasing identical complexes, the immune response will also increase. However, this process will continue until a certain point, after which it will decrease and may not be observed at all later. This phenomenon was studied in the process of using conjugated antigens with a different number 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 mechanism of activation of B-group lymphocytes.

Cancer-embryonic antigen

It is one of the types of proteins of normal tissue, which in healthy people is produced in a small amount by the cells of certain organs. REA in its chemical structure is a combination of carbohydrates and protein. Its appointment 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 important tasks. They are associated with the stimulation of cellular reproduction. Cancer-embryonic antigen is detected in the tissues of digestive organs, but in a fairly small amount. The name of this oncomarker partly characterizes its biological nature, but for the most part still properties that are valuable in laboratory research. The term "embryonic" has a connection with physiological tasks during development during the antenatal period, "antigen" indicates the possibility of identifying it in biological media using the immunochemical binding method. In this case, it does not show any properties directly in the body. Normally, in a healthy organism, the REA concentration 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 oncomarker.

Level of REA

The antigen test is used in the diagnosis of various malignant tumors, mainly cancer of the rectum and large intestine. 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 cancer of the colon and rectum test is the highest sensitivity. This is what makes it possible to apply it in the initial diagnosis. After the successful completion of the operation to remove the entire tumor tissue, the CEA concentration returns to normal after a maximum of two months. Regular analyzes later make it possible to evaluate a patient's condition after receiving treatment. Detection of high level of CEA allows timely detection of relapse of pathology. With a decrease in the antigen content against the background of therapy, experts conclude that the effect of the therapeutic effect.

Increase in CEA concentration: a range of pathologies

However, the test is not considered specific for tumors. Raising the level of CEA can be noted against a background of various diseases of internal organs that are inflammatory and of a different nature. In 20-50% of patients with benign pathologies of the pancreas, intestines, lungs and liver, the concentration of the antigen slightly increases. The same is observed against cirrhosis, chronic hepatitis, ulcerative colitis, cystic fibrosis, emphysema, bronchitis, Crohn's disease, pancreatitis, pneumonia, autoimmune diseases, tuberculosis. In addition, the increase in the level may be due not to the disease, but, for example, the regular intake of alcohol or smoking.

Features of blood transfusion

The main one is the specificity and individuality that the erythrocyte antigens possess. If the biopolymers of the recipient and donor are incompatible, the blood transfusion is strictly forbidden. Otherwise, the pathological processes and even the death of the patient are inevitable. In immunogenetics, serological tests 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 its individual chemical composition and its own structure.