The human lymphatic system performs a number of important protective functions that prevent the development of pathogenic microorganisms or viruses in liquid media, cells and tissues. B-lymphocytes are responsible for humoral immunity, which, with further maturation, synthesize immunoglobulins (Ig). The structure of these substances allows you to find, mark and destroy antigens that have entered the body. What are the characteristics of molecules?
Plasma cells
All lymphatic cells of the human body are divided into two large groups: T-lymphocytes and B-lymphocytes. The first are responsible for cellular immunity, absorbing antigens in the process of phagocytosis. The task of the latter is to synthesize specific antibodies - humoral immunity.
B-lymphocytes are determined in secondary lymphoid organs (lymph nodes, spleen), and then form a population of plasma cells, which are also called plasma cells. They further migrate to the red bone marrow, mucous membranes and tissues.
Plasmocytes reach large sizes (up to 20 microns), stain basophilically, i.e. in purple with the help of dyes. In the centerof these cells is a large nucleus with characteristic clumps of heterochromatin, which resemble the spokes of a wheel.
The cytoplasm stains lighter than the nucleus. It houses a powerful transport center, consisting of the endoplasmic reticulum and the Golgi apparatus. AH is quite strongly developed, forming the so-called light courtyard of the cell.
All of these structures are aimed at the synthesis of antibodies that are responsible for humoral immunity. The structure of the immunoglobulin molecule has its own characteristics, so the gradual and high-quality maturation of these structures in the process of synthesis is important.
Actually, this is why such a dense network of EPS and the Golgi apparatus has been developed. Also, the genetic apparatus of plasma cells, enclosed in the nucleus, is mainly aimed at the synthesis of antibody proteins. Mature plasma cells are an example of a high degree of determination, so they rarely divide.
The structure of immunoglobulin antibodies
These highly specialized molecules are glycoproteins because they have protein and carbohydrate parts. We are interested in the skeleton of immunoglobulins.
A molecule consists of 4 peptide chains: two heavy (H-chains) and two light (L-chains). They connect to each other through disulfide bonds, and as a result, we can observe the shape of the molecule, resembling a slingshot.
The structure of immunoglobulins is aimed at connecting with antigens using specific Fab-fragments. At the free ends of the “slingshot”, each such region is formed by two variable domains: one from heavy andone from the light chain. Permanent domains serve as a scaffold (3 on each heavy and one on light chains).
Mobility of the variable ends of immunoglobulin is provided by the presence of a hinge area in the place where a disulfide bond is formed between two H-chains. This greatly simplifies the process of antigen-antibody interaction.
The third end of the molecule, which does not interact with foreign molecules, remains unconsidered. It is called the Fc region and is responsible for the attachment of immunoglobulin to the membranes of plasma cells and other cells. By the way, light chains can be of two types: kappa (κ) and lambda (λ). They are interconnected by disulfide bonds. There are also five types of heavy chains, according to which different types of immunoglobulins are classified. These are α-(alpha), δ-(delta), ε-(epsilon), γ-(gamma) Μ-(mu) chains.
Some antibodies are able to form polymer structures that are stabilized by additional J-peptides. This is how dimers, trimers, tetramers or pentomers of Ig of a certain type are formed.
Another additional S-chain is characteristic of secretory immunoglobulins, the structure and biochemistry of which allow them to function in the mucous membranes of the mouth or intestines. This extra chain prevents natural enzymes from breaking down antibody molecules.
The structure and classes of immunoglobulins
The variety of antibodies in our body predetermines the variability of humoral immunity functions. Each Ig classhas its own distinctive characteristics, by which it is not difficult to guess their role in the immune system.
The structure and functions of immunoglobulins are directly dependent on each other. At the molecular level, they differ in the amino acid sequence of the heavy chain, the types of which we have already mentioned. Therefore, there are 5 types of immunoglobulins: IgG, IgA, IgE, IgM and IgD.
Features of immunoglobulin G
IgG does not form polymers and does not integrate into cell membranes. The presence of a gamma-heavy chain was revealed in the composition of the molecules.
A distinctive feature of this class is the fact that only these antibodies are able to penetrate the placental barrier and form the immune defense of the fetus.
IgG makes up 70-80% of all serum antibodies, so the molecules are easily detected by laboratory methods. In the blood, 12 g / l is the average content of this class, and this figure is usually reached by the age of 12.
The structure of immunoglobulin G allows you to perform the following functions:
- Detoxification.
- Opsonization of antigens.
- Starting complement-mediated cytolysis.
- Presentation of antigen to killer cells.
- Ensuring the immunity of the newborn.
Immunoglobulin A: features and functions
This class of antibody occurs in two forms: serum and secretory.
In the blood serum, IgA makes up 10-15% of all antibodies, and its average amountis 2.5 g/l by 10 years of age.
We are more interested in the secretory form of immunoglobulin A, since about 60% of the molecules of this class of antibodies are concentrated in the mucous membranes of the body.
The structure of immunoglobulin A is also distinguished by its variability due to the presence of a J-peptide, which can participate in the formation of dimers, trimers or tetramers. Due to this, one such antibody complex is able to bind a large number of antigens.
During the formation of IgA, another component is attached to the molecule - the S-protein. Its main task is to protect the entire complex from the destructive action of enzymes and other cells of the human lymphatic system.
Immunoglobulin A is found in the mucous membranes of the gastrointestinal tract, genitourinary system and respiratory tract. IgA molecules envelop antigenic particles, thereby preventing their adhesion to the walls of hollow organs.
The functions of this class of antibodies are as follows:
- Neutralization of antigens.
- The first barrier of all humoral immunity molecules.
- Opsonize and label antigens.
Immunoglobulin M
Representatives of the IgM class are distinguished by large molecular sizes, since their complexes are pentamers. The whole structure is supported by a J-protein, and the backbone of the molecule is the heavy chains of the nu-type.
The pentameric structure is characteristic of the secretory form of this immunoglobulin, but there are also monomers. The latter are attached to membranesB-lymphocytes, thereby helping cells to detect pathogenic elements in body fluids.
Only 5-10% is IgM in blood serum, and its content on average does not exceed 1 g/l. Antibodies of this class are the oldest in evolutionary terms, and they are synthesized only by B-lymphocytes and their precursors (plasmocytes are not capable of this).
The number of antibodies M increases in newborns, because. this is a factor in the intense secretion of IgG. Such stimulation has a positive effect on the development of the baby's immunity.
The structure of immunoglobulin M does not allow it to cross the placental barriers, so the detection of these antibodies in fetal fluids becomes a signal of a violation of metabolic mechanisms, an infection or a defect in the placenta.
IgM functions:
- Neutralization.
- Opsonization.
- Activation of complement-dependent cytolysis.
- Formation of immunity of the newborn.
Features of immunoglobulin D
This type of antibody has been little studied, so their role in the body is not fully understood. IgD occurs only in the form of monomers; in blood serum, these molecules make up no more than 0.2% of all antibodies (0.03 g/l).
The main function of immunoglobulin D is reception within the membrane of B-lymphocytes, but only 15% of the entire population of these cells have IgD. Antibodies are attached using the Fc-terminus of the molecule, and heavy chains belong to the delta class.
Structure and functionsimmunoglobulin E
This class makes up a small fraction of all serum antibodies (0.00025%). IgE, also known as reagin, are highly cytophilic: the monomers of these immunoglobulins are attached to the membranes of mast cells and basophils. As a result, IgE affects the production of histamine, which leads to the development of inflammatory reactions.
Epsilon-type heavy chains are present in the structure of immunoglobulin E.
Because of the small amount, these antibodies are very difficult to detect by laboratory methods in blood serum. Elevated IgE is an important diagnostic sign of allergic reactions.
Conclusions
The structure of immunoglobulins directly affects their functions in the body. Humoral immunity plays a big role in maintaining homeostasis, so all antibodies must work clearly and smoothly.
The contents of all Ig classes are strictly defined for humans. Any changes recorded in the laboratory may be the reason for the development of pathological processes. This is what doctors use in their practice.
