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, upon further maturation, synthesize immunoglobulins (Ig). The structure of these substances allows you to find, label and destroy antigens that have come into the body. What are the features of the molecules?
Plasma cells
All lymphatic cells of the human body are divided into two large groups: T-lymphocytes and B-lymphocytes. The former are responsible for cellular immunity, absorbing antigens during phagocytosis. The second task is the synthesis of specific antibodies - humoral immunity.
B-lymphocytes are determined in the secondary lymphoid organs (lymph nodes, spleen), and then form a population of plasmocytes, 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 μm), stained basophilically, i.e., in purple using dyes. At the center of these cells is a large nucleus with characteristic clumps of heterochromatin that resemble wheel spokes.
The cytoplasm stains lighter than the nucleus. It houses a powerful transport center, consisting of an endoplasmic reticulum and a Golgi apparatus. AH is developed quite strongly, forming the so-called bright 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 peculiarities; therefore, the gradual and high-quality maturation of these structures during synthesis is important.
Actually, for this, such a dense network of EPS and Golgi apparatus is developed. Also, the genetic apparatus of plasmocytes contained in the nucleus is aimed primarily 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 antibodies of immunoglobulin
These highly specialized molecules are glycoproteins, because they have protein and carbohydrate moieties. We are interested in the skeleton of immunoglobulins.
The molecule consists of 4 peptide chains: two heavy (H-chains) and two light (L-chains). They are connected to each other by means of disulfide bonds, and as a result, we can observe the shape of a molecule resembling a slingshot.
The structure of immunoglobulins is aimed at combining with antigens using specific Fab fragments. At the free ends of the slingshot, each such site is formed by two variable domains: one from the heavy and one from the light chain. The framework is composed of permanent domains (3 on each heavy and one on light chains).
The mobility of the variable ends of the immunoglobulin is ensured by the presence of a hinge in the place where the disulfide bond between the two H-chains is formed. This greatly simplifies the antigen-antibody interaction process.
The third end of the molecule, which does not interact with foreign molecules, remains unexamined. It is called the Fc region and is responsible for the attachment of immunoglobulin to the membranes of plasmocytes 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 by which various types of immunoglobulins are classified. These are the α- (alpha), δ- (delta), ε- (epsilon), γ- (gamma) μ- (mu) chains.
Some antibodies are capable of forming polymer structures that are stabilized by additional J peptides. Thus dimers, trimers, tetramers or pentomeres of a certain type of Ig 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 oral cavity or intestines. This additional chain prevents the destruction of antibody molecules by natural enzymes.
The structure and classes of immunoglobulins
The variety of antibodies in our body determines the variability of the functions of humoral immunity. Each Ig class has its own distinctive characteristics, according to which it is easy 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, 5 types of immunoglobulins are distinguished: 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 detected 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 embryo.
IgG accounts for 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 such an indicator is usually reached by the age of 12.
The structure of immunoglobulin G allows you to perform the following functions:
- Neutralization of toxins.
- Opsonization of antigens.
- Launch of complement-mediated cytolysis.
- Presentation of antigen to killer cells.
- Ensuring the immunity of the newborn.
Immunoglobulin A: features and functions
This class of antibodies occurs in two forms: serum and secretory.
In serum, IgA accounts for 10-15% of all antibodies, and its average amount is 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 that can participate in the formation of dimers, trimers or tetramers. Due to this, one such complex of antibodies is able to bind a large number of antigens.
During the formation of IgA, another component is attached to the molecule - 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.
- They are the first barrier among all molecules of humoral immunity.
- Opsonate and label antigens.
Immunoglobulin M
Representatives of the IgM class are distinguished by the large size of the molecule, because their complexes are pentamers. The entire structure is supported by the J-protein, and nude-type heavy chains are the backbone of the molecule.
The pentameric structure is characteristic of the secretory form of this immunoglobulin, however, monomers also exist. The latter are attached to the membranes of B-lymphocytes, thereby helping cells detect pathogenic elements in body fluids.
Only 5-10% is IgM in 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 predecessors (plasmocytes are not capable of this).
The number of antibodies M increases in newborns, because this is a factor in intense IgG secretion. Such stimulation has a positive effect on the development of the infant's immunity.
The structure of immunoglobulin M does not allow penetration through placental barriers, therefore, the detection of these antibodies in fetal fluids becomes a signal of metabolic disturbance, infection or a defect in the placenta.
IgM functions:
- Neutralization.
- Opsonization.
- Activation of complement dependent cytolysis.
- The formation of immunity of the newborn.
Features of immunoglobulin D
This type of antibody has not been sufficiently studied, so their role in the body is not fully understood. IgDs are found only in the form of monomers; in the 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 the reception of B-lymphocytes in the membrane, but only 15% of the entire population of these cells have IgD. Antibodies are attached using the Fc end of the molecule, and the heavy chains belong to the delta class.
The structure and functions of immunoglobulin E
This class accounts for a small fraction of all serum antibodies (0,00025%). IgE, also known as reagin, is highly cytophilic: monomers of these immunoglobulins attach 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.
Due to 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 large role in maintaining homeostasis, so all antibodies must work clearly and smoothly.
The content of all classes of Ig is strictly defined for humans. Any changes recorded in the laboratory may be the reason for the development of pathological processes. Doctors use this in their practice.