Special substances genetically alien to us that provoke the body's immune response through the activation of specific B and / or T lymphocytes are called antigens. The properties of antigens imply their interaction with antibodies. Almost any molecular structure can cause this reaction, for example: proteins, carbohydrates, lipids, etc.
Most often, they become bacteria and viruses, which every second of our lives try to get inside the cells in order to transmit and multiply their DNA.
Structure
Foreign structures are usually high molecular weight polypeptides or polysaccharides, but other molecules, such as lipids or nucleic acids, can also perform their functions. Smaller formations become this substance if they combine with a larger protein.
Antigens are combined with the antibody. The combination is very similar to the analogy of the lock and key. Each Y-shaped antibody molecule has at least two binding regions that can attach to a specific site on the antigen. The antibody is able to connect with the same parts of two different cells at the same time, which can lead to aggregation of neighboring elements.
The structure of antigens consists of two parts: information and carrier. The first determines the specificity of the gene. It is responsible for certain sections of the protein called epitopes (antigenic determinants). These are fragments of molecules that provoke immunity to response, forcing it to defend itself and produce antibodies with similar characteristics.
The supporting part helps the substance to penetrate into the body.
Chemical origin
- Proteins Antigens are usually large organic molecules that are proteins or large polysaccharides. They do their job well because of their high molecular weight and structural complexity.
- Lipids. They are considered inferior due to their relative simplicity and lack of structural stability. However, when they attach to proteins or polysaccharides, they can act as complete substances.
- Nucleic acids. Poor fit for the role of antigens. The properties of antigens are absent in them due to relative simplicity, molecular flexibility, and rapid decay. Antibodies to them can be produced by artificially stabilizing them and binding to an immunogenic carrier.
- Carbohydrates (polysaccharides). By themselves, they are too small to function independently, but in the case of antigens of the erythrocyte blood group, protein or lipid carriers can contribute to the required size, and the polysaccharides present in the form of side chains give immunological specificity.
Main characteristics
To be called an antigen, a substance must possess certain properties.
First of all, it must be alien to the organism where it seeks to get. For example, if a transplant recipient receives a donor organ with several major differences in HLA (human leukocyte antigen), the organ is perceived as foreign and subsequently rejected by the recipient.
The second function of antigens is immunogenicity. That is, a foreign substance should be perceived by the immune system as an aggressor upon penetration, cause a response and cause it to produce specific antibodies that can destroy the invader.
Many factors are responsible for this quality: the structure, the weight of the molecule, its speed, etc. An important role is played by how foreign it is to the individual.
The third quality is antigenicity - the ability to provoke a reaction in certain antibodies and adhere to them. Epitopes are responsible for this, and the type to which the hostile microorganism belongs depends on them. This property makes it possible to bind to T-lymphocytes and other attacking cells, but cannot cause the immune response itself.
For example, particles with a lower molecular weight (haptens) are able to bind to the antibody, but for this they must be attached to the macromolecule as a carrier to trigger the reaction itself.
When antigen-bearing cells (such as red blood cells) are transferred from a donor to a recipient, they can be immunogenic just like the external surfaces of bacteria (capsule or cell wall), as well as the surface structures of other microorganisms.
Colloidal state and solubility are essential properties of antigens.
Complete and incomplete antigens
Depending on how well they perform their functions, these substances are of two types: complete (consisting of protein) and incomplete (haptens).
A complete antigen is capable of possessing immunogenicity and antigenicity at the same time, inducing the formation of antibodies and entering into specific and observable reactions with them.
Haptens are substances that cannot, due to their tiny size, affect immunity and therefore must merge with large molecules so that they can deliver them to the "crime scene." In this case, they become full, and the hapten part is responsible for specificity. Determined by in vitro reactions (in vitro studies).
Such substances are known as foreign or non-independent, and those that are present on the body's own cells are called auto- or self-antigens.
Specificity
- Species - is present in living organisms belonging to the same species and having common epitopes.
- Typical - it happens with completely different creatures. For example, this is the identity between staphylococcus and human connective tissue or red blood cells and plague bacillus.
- Pathological - possible with irreversible changes at the cellular level (for example, from radiation or drugs).
- Stadiospecific - is produced only at some stage of existence (in the fetus during fetal development).
Autoantigens begin to be produced during malfunctions when the immune system recognizes certain parts of its own body as foreign and tries to destroy them by synthesis with antibodies. The nature of such reactions has not yet been precisely determined, but leads to such terrible incurable diseases as vasculitis, SLE, multiple sclerosis, and many others. In diagnosing these cases, in vitro studies are needed that find the raging antibodies.
Blood types
On the surface of all blood cells there are a huge number of different antigens. All of them are combined thanks to special systems. In total there are more than 40.
The erythrocyte group is responsible for blood compatibility during transfusion. It includes, for example, the serological system ABO. All blood groups have a common antigen - H, which is a precursor to the formation of substances A and B.
In 1952, a very rare example was reported from Mumbai in which antigens A, B, and H were absent on red blood cells. This blood type was called "Bombay" or "fifth." Such people can only take blood from their own group.
Another system is the Rhesus factor. Some Rh antigens are structural components of an erythrocyte membrane (RBC). If they are absent, then the membrane is deformed and leads to hemolytic anemia. In addition, rhesus is very important during pregnancy and its incompatibility in mother and child can lead to big problems.
When antigens are not part of the membrane structure (for example, A, B and H), their absence does not affect the integrity of red blood cells.
Antibody interaction
It is possible only on condition that the molecules of both are close enough for some of the individual atoms to fit into complementary depressions.
The epitope is the corresponding region of antigens. The properties of antigens allow most of them to have several determinants; if two of them or more are identical, then such a substance is considered multivalent.
Another way to measure interaction is the avidity of binding, which reflects the overall stability of the complex of antibodies and antigens. It is defined as the total binding strength of all its places.
Antigen Representative Cells (APC)
Those that can absorb the antigen and deliver it to the desired location. There are three types of these representatives in our body.
- Macrophages. Usually at rest. Their phagocytic capabilities increase significantly when they are stimulated to go into an active form. Along with lymphocytes, they are present in almost all lymphoid tissues.
- Dendritic cells. They are characterized by long cytoplasmic processes. Their main role is to act as antigen fishers. They are non-phagocytic in nature and are located in the lymph nodes, thymus, spleen and skin.
- B lymphocytes. On the surface of the molecule are released molecules of an intram Membrane immunoglobulin (Ig), which function as receptors of cellular antigens. The properties of antigens allow them to bind only one type of foreign substance. This makes them much more effective than macrophages, which should absorb any foreign material that comes in their way.
The descendants of B cells (plasma cells) produce antibodies.