What is immunoglobulin ?
Immunoglobulins (abbreviated to Ig), also antibodies are proteins that are produced by humans and other vertebrates in response to antigens. These are foreign substances such as viruses, bacteria or larger molecules. Because the antibodies bind to foreign substances, these can be defused.
Some antibodies come as separate molecules in the blood and other body fluids. Other white blood cells and are attached to act as a sense of the respective cell.
If the antigen comes in contact with the appropriate B-cell is stimulated to divide and produce large quantities of antibodies. The antibodies then end up in the blood and in the extracellular fluid of the tissues. In addition to the antibody-producing Plasma Cells arise also memory cells, the B-memory cells. In recent years, remain in the body in case the antigen will come back. By this mechanism, the body can keep out within a reasonable time against all kinds of antigens, all antigens without the need to be continuously available. Related vaccination.
Immunoglobulins are among the globulin fraction of the blood and are an important part of the immune system.
General construction of immunoglobulins
An antibody consists of two identical heavy and two identical light chains amino acid which are held together by covalent (disulfide bonds) and non-covalent bonds (hydrogen bonds, ionic bonds, van der Waals forces and hydrophobic gasket). In each of these chains, there is an unchanging (constant) part, and a changeable (variable) part. The constant part is responsible for the stability and the interaction with endogenous receptors (immune) cells. The variable part (CDR 1 + 2 + 3) binds the antigen.
By means of enzymes, the molecule can be an antibody to be split in three parts: two Fab fragments (these are the antigen-binding portions), and an Fc portion. The Fab fragments consist of the light and a portion of the heavy chain. The Fc portion consists of a part of both heavy chains.
Basis for variation
The genes for immunoglobulins are divided into V, D and J segments. From each of these segments, a number of different copies present in the genome. Thus there are in humans for the heavy chain for over 50 V segments, 25 D segments, and 6 J segments are known. Of each type will be sent by B-cell (and therefore per immunoglobulin) a gene chosen by enzymes (RAG-1 and RAG-2) to form with it a heavy chain. Together with the combined genes for the light chain (from different V and J segments), this is the basis for the wide variety of immunoglobulins. This is called recombination and provides approximately 2.5 * 10 ^ 5 possibilities for the formation of an immunoglobulin molecule. Further variation is obtained by, inter alia, insertion and deletion of nucleotides. Consequently, there are finally some 10 ^ 11 possibilities for a unique immunoglobulin. Immunoglobulins also make themselves after their interaction with antigen better fit.
A certain B-cell thus makes only one type of immunoglobulin which is becoming the same is built up. Because there are two chromosomes, there thereof per B-cell single-handedly switched off, in order to prevent that a B-cell allows two different types of immunoglobulins. This is known as allelic exclusion.
Immunoglobulin types
There are several forms: immunoglobulin M (IgM), G (IgG), A (IgA), E (IgE), and D (IgD). They are all made by B-lymphocytes, but under different circumstances. A young B-cell produces, in principle, IgM and IgD. After activation by a T-helper cell it undergoes a process which is called class switching and produces IgA, IgG or IgE. When the class switching changes the constant part of the chain of the immunoglobulin, and not the variable part to which the antigen binds. Class switching effect, therefore, does not require the B-cell suddenly goes produce antibody against a different antigen. In class switching takes place deletion of genetic material, it is irreversible.
IgG (Immunoglobulin G)
IgG is produced in larger amounts or in a repeated contact with the antigen. The IgG molecule can be regarded as a typical antibody. Within the light chain, there are two disulfide bridges, one in the variable region and one in the constant region. There are four of these bridges in the heavy chain, which is two times as long as the light chain. Each disulfide compound forms a peptide loop of 60 to 70 amino acid residues; if the amino acid sequences of these loops are compared, is a high degree of homology on Gay. This implies that each immunoglobulin-peptide chain consists of series of globular areas with a very similar secondary and tertiary structure (folding). IgG is divided into four subclasses: IgG1, IgG2, IgG3 and IgG4. The 4 subclasses of human IgG differ little from each other in the amino acid sequence. IgG can reach through the placenta of pregnant women to the fetus and causes in the first six months for the defense of the baby.
IgM (Immunoglobulin M)
Human IgM is generally a pentamer of the four-chain unit. The subunits of the pentamer are connected by disulfide bridges. The complete molecule consists of a densely packed central area with projecting arms. This is visible with electron microscopy. IgM is the first time at an infection often produced antibody which is later 'redeemed' by IgG, and with its ten an excellent immunoglobulin binding sites for the capture of antigens.
IgA (Immunoglobulin A)
IgA is found in the amino acid residues. The amino acid residues are divided into four domains. A feature that is shared with IgM, an additional peptide of 18 residues. Allows an additional chain may be covalently bonded to create a dimer. IgA is found mainly in the stomach, intestines, saliva and milk.
IgD (Immunoglobulin D)
Less than 1% of the total immunoglobulin in serum is IgD. The protein is more susceptible to proteolysis than IgG1, IgG2, IgA, and IgM, and has a tendency to spontaneous proteolysis.
IgE (Immunoglobulin E)
IgE is sitting on the mucous membranes and is usually fixed with its Fc portion to the Fc receptor of basophils which release histamine by binding with antigen. This is also the cause of allergic reactions, which are characterized by example, red eyes and a red nose because of the vasodilatieve (vasodilator) effects of histamines. By the excessive release of histamine can be anti-allergic agents (so-called anti-histamines) take that counteract the action of histamine.
Action
Immunoglobulins bind to an epitope of the antigen. This can have several effects:
- Neutralization. This means that the antibodies bind to certain parts of the antigen, whereby the antigen no interactions with cells or molecules may be more interested in. The antigen thereby loses its effect. This is done, inter alia, to toxins (e.g., produced by bacteria, so-called exotoxins) or viruses.
- Opsonization. By surrounding antigen with antibodies is phagocytosis (as it were, the eating of cells) facilitated. This is because phagocytic cells possess receptors for the constant portion of immunoglobulin (Fc receptors).
- The complement system is activated by the Fc portion. This leads to improvement of opsonization, because phagocytic cells also possess complement receptors. As a result, phagocytosis even further facilitated. Also, the binding of complement may lead to immediate destruction of the antigen.
- Cell-mediated cytotoxicity is dependent on antibodies. This is also known under the abbreviation ADCC, which stands for antibody dependent cell-mediated cytotoxicity. This is the destruction of the antigen by NK-cells. These cells also possess Fc-receptors and by binding to antibodies, they give off certain substances that kill the antigen.
Immunoglobulins determine in a medical lab
In a medical laboratory, the amount of the various immunoglobulins are determined.
IgG
IgG is the largest fraction of the immunoglobulins in the human body. There are 4 sub classes, IgG1, IgG2, IgG3 and IgG4. These subclasses have different biological properties. For example, to activate the subclasses differ in their capacity in the complement system. IgG1 and IgG3 this may be fine, IgG2 and IgG4 to a lesser degree or not, respectively. Elevated levels of subclassing may occur in case of chronic inflammation, auto-immune diseases and infections, or in multiple myeloma or B cell lymphoma. An example of this is IgG4-related disease.
IgM
An increase in IgM is related to infections, liver disease, Waldenstrom's, or at an IgM plasmacytoma. Moderate increases may also occur with lymphoproliferative diseases, autoimmune diseases and carcinomas.
IgA
The main function of IgA is to protect the body against penetration of a variety of pathogens by the presence in the tear fluid, colostrum, and bile of the glands of air and urinary tract and gastrointestinal tract. In the medical laboratory, the amount of IgA can be determined in order to demonstrate a possible IgA deficiency. This is of interest because these individuals have a better chance of rheumatic and infectious disease, or an increased risk of anti-IgA which can cause problems in transfusion of blood products. In blood, increases in IgA can be found after infections, increases in IgA in people with chronic or acute leukemias, solid tumors, and autoimmune diseases or to a monoclonal gammopathy, non-Hodgkin's lymphoma and certain forms of amyloidosis.