Special substances, genetically alien to us, which 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 are bacteria and viruses, which every second of our lives try to get inside the cells in order to transfer 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 are combined with a larger protein.
Antigens match with an antibody. The combination is very similar to the lock and key analogy. Each Y-shaped antibody molecule has at leastat least two binding regions that can attach to a specific site on an antigen. The antibody is able to bind to 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: informational and carrier. The first determines the specificity of the gene. Certain sections of the protein, called epitopes (antigenic determinants), are responsible for it. These are fragments of molecules that provoke the immune system to respond, forcing it to defend itself and produce antibodies with similar characteristics.
The carrier part helps the substance to penetrate the body.
Chemical origin
- Proteins. Antigens are usually large organic molecules that are proteins or large polysaccharides. They do an excellent job due to their high molecular weight and structural complexity.
- Lipids. Considered inferior due to their relative simplicity and lack of structural stability. However, when attached to proteins or polysaccharides, they can act as complete substances.
- Nucleic acids. Poorly suited to 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 artificial stabilization and binding to an immunogenic carrier.
- Carbohydrates (polysaccharides). By themselves too small to functionon their own, but in the case of erythrocyte blood group antigens, protein or lipid carriers may contribute to the required size, and polysaccharides present as side chains confer immunological specificity.
Key Features
To be called an antigen, a substance must have certain properties.
First of all, it must be alien to the organism it seeks to enter. For example, if a transplant recipient receives a donor organ with several major HLA (human leukocyte antigen) differences, 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 when it penetrates, cause a response and force it to produce specific antibodies that can destroy the invader.
Many factors are responsible for this quality: the structure, 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 cause a reaction in certain antibodies and link with them. Epitopes are responsible for this, and it is on them that the type to which the hostile microorganism belongs depends. This property allows it to bind to T-lymphocytes and other attacking cells, but cannot elicit an immune response itself.
For example, lower molecular weight particles(haptens) are able to bind to an antibody, but for this they must be attached to the macromolecule as a carrier to start the reaction itself.
When antigen-bearing cells (such as red blood cells) from a donor are transfused into a recipient, they can be immunogenic in the same way as the outer surfaces of bacteria (capsule or cell wall) and the surface structures of other microorganisms.
Colloid 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 full antigen is able to be immunogenic and antigenic at the same time, induce the formation of antibodies and enter into specific and observable reactions with them.
Haptens are substances that, due to their tiny size, cannot affect the immune system and therefore must merge with large molecules so that they can be delivered to the “crime scene”. In this case, they become complete, and the hapten part is responsible for specificity. Determined by in vitro reactions (research done in a laboratory).
Such substances are known as foreign or non-self, and those present on the body's own cells are called auto- or self-antigens.
Specificity
- Species - present in living organisms,belonging to the same species and having common epitopes.
- Typical - happens to completely dissimilar creatures. For example, this is the identity between staphylococcus and human connective tissues, or red blood cells and plague bacillus.
- Pathological - possible with irreversible changes at the cellular level (for example, from radiation or drugs).
- Stage-specific - produced only at some stage of existence (in the fetus during fetal development).
Autoantigens begin to be produced in case of failures, when the immune system recognizes certain parts of its own body as foreign and tries to destroy them by synthesizing with antibodies. The nature of such reactions is still not exactly established, but leads to such terrible incurable diseases as vasculitis, SLE, multiple sclerosis and many others. In the diagnosis of these cases, in vitro studies are needed that find rampant antibodies.
Blood types
On the surface of all blood cells there is a huge number of different antigens. All of them are united thanks to special systems. There are more than 40 in total.
The erythrocyte group is responsible for the compatibility of blood during transfusion. It includes, for example, the ABO serological system. All blood groups have a common antigen - H, which is the precursor to the formation of substances A and B.
In 1952, a very rare example was reported from Mumbai in which the antigens A, B and Habsent from red blood cells. This blood type was called "Bombay" or "fifth". Such people can only accept blood from their own group.
Another system is the Rh factor. Some Rh antigens represent structural components of the erythrocyte membrane (RBC). If they are absent, then the shell is deformed and leads to hemolytic anemia. In addition, Rh is very important during pregnancy and its incompatibility between mother and child can lead to big problems.
When antigens are not part of the membrane structure (such as A, B and H), their absence does not affect the integrity of the red blood cells.
Interaction with antibodies
Only possible if the molecules of both are close enough for some of the individual atoms to fit into complementary cavities.
An epitope is the corresponding region of antigens. The properties of antigens allow most of them to have multiple determinants; if two or more of them 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 antibody/antigen complex. It is defined as the total binding strength of all its places.
Antigen presenting cells (APC)
Those that can absorb the antigen and deliver it to the right place. There are three types of these representatives in our body.
- Macrophages. They are usually at rest. Their phagocytic capabilitiesincrease significantly when they are stimulated to become active. They are present along with lymphocytes in almost all lymphoid tissues.
- Dendritic cells. Characterized by long-term cytoplasmic processes. Their primary role is to act as antigen scavengers. They are non-phagocytic in nature and are found in the lymph nodes, thymus, spleen and skin.
B-lymphocytes. They secrete intramembrane immunoglobulin (Ig) molecules on their surface, which function as receptors for cellular antigens. The properties of antigens allow them to bind only one type of foreign substance. This makes them much more efficient than macrophages, which must devour any foreign material that gets in their way
Descendants of B cells (plasma cells) produce antibodies.
