Antibody
Encyclopedia : A : AN : ANT : Antibody
Definition
Picture: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/ANTIBODY.gifImmunoglobulins are glycoproteins in the immunoglobulin superfamily that function as antibodies. The terms antibody and immunoglobulin are often used interchangeably. They are found in the blood and tissue fluids, as well as many secretions. In structure, they are globulins (in the γ-region of protein electrophoresis). They are synthesized and secreted by plasma cells that are derived from the B cells of the immune system. B cells are activated upon binding to their specific antigen and differentiate into plasma cells. In some cases, the interaction of the B cell with a T helper cell is also necessary.
There are five types: IgA, IgD, IgE, IgG, and IgM. (Ig stands for immunoglobulin, which is another name for antibody)
This is according to differences in their heavy chain constant domains. (The isotypes are also defined with light chains, but they do not define classes, so they are often neglected.) Other immune cells partner with antibodies to eliminate pathogens depending on which IgG, IgA, IgM, IgD, and IgE constant binding domain receptors it can express on its surface.
The antibodies that a single B lymphocyte produces can differ in their heavy chain, and the B cell often expresses different classes of antibodies at the same time. However, they are identical in their specificity for antigen, conferred by their variable region. To achieve the large number of specificities the body needs to protect itself against many different foreign antigens, it must produce millions of B lymphoyctes. It is important to note that, in order to produce such a diversity of antigen binding sites with a separate gene for each possible antigen, the immune system would require many more genes than exist in the genome. Instead, as Susumu Tonegawa showed in 1976, portions of the genome in B lymphocytes can recombine to form all the variation seen in the antibodies and more. Tonegawa won the Nobel Prize in Physiology or Medicine in 1987 for his discovery.
Structure of the antibody
Immunoglobulins are heavy plasma proteins, often with added sugar chains (see glycosylation) on N-terminal (all antibodies) and occasionally O-terminal (IgA1 and IgD) amino acid residues.The basic unit of each antibody is a monomer. An antibody can be monomeric, dimeric, trimeric, tetrameric, pentameric, etc. The monomer is a "Y"-shape molecule that consists of four polypeptide chains: two identical heavy chains and two identical light chains connected by disulfide bonds.
Heavy Chain
There are five types of heavy chain: γ, δ, α, μ and ε. They define classes of immunoglobulins. Heavy chains α and γ have approximately 450 amino acids, while μ and ε have approximately 550 amino acids. Each heavy chain has a constant region, which is the same by all immunoglobulins of the same class, and a variable region, which differs between immunoglobulins of different B cells, but is the same for all immunoglobulins produced by the same B cell. Heavy chains γ, α and δ have the constant region composed of three domains but have a hinge region; the constant region of heavy chains μ and ε is composed of four domains. The variable domain of any heavy chain is composed of one domain. These domains are about 110 amino acids long. There are also some amino acids between constant domains.
Light Chain
There are only two types of light chain: λ and κ. In humans, they are similar, but only one type is present in each antibody. Each light chain has two successive domains: one constant and one variable domain. The approximate length of a light chain is from 211 to 217 amino acids.
The "Y"-shaped monomer has two heavy and two light chains. Together this gives six to eight constant domains and four variable domains. Each half of the forked end of the "Y" is called an Fab fragment. It is composed of one constant and one variable domain of each the heavy and the light chain, which together shape the antigen binding site at the amino terminal end of the monomer. The two variable domains bind their specific antigens.
The enzyme papain cleaves a monomer into two Fab (fragment antigen binding) fragments and an Fc (fragment crystallizable) fragment. The enzyme pepsin cleaves below hinge region, so a f(ab)2 fragment and a fc fragment is formed.
Modular Design
Together, the antibodies in an organism can bind a wide variety of foreign antigens. It is estimated that your body has an antibody designed to take on any protein in the Universe. However, this would take up half of your genes in every cell to create just your antibodies. Therefore, scientists knew that there was a way for your body to "compress" all the info needed to make your antibodies: somatic recombination.
Somatic recombination is when genes are selected (variable (V), diversity (D) and joining (J) for heavy chains, and only V and J for light chains) to form countless combinations. The main reason that the human immune system is capable of binding so many antigens is the variable region of the heavy chain. To be specific, it is the area where these V, D and J genes are found - otherwise known as the complementarity determining region 3 (CDR3).
Fc region
The Fc fragment, the stem of the "Y," is composed of two heavy chains that each contribute two to three constant domains (depending on the class of the antibody). Fc binds to various cell receptors and complement proteins. In this way, it mediates different physiological effects of antibodies (opsonization, cell lysis, mast cell, basophil and eosinophil degranulation and other processes).
The variable regions of the heavy and light chains can be fused together to form a single chain variable fragment (scFv), which retains the original specificity of the parent immunoglobulin.
A crude estimation of immunoglobulin levels can be made by protein electrophoresis. Here the plasma proteins are separated into albumin, alpha-globulins (1 and 2), beta-globulins (1 and 2) and gamma-globulins according to weight. Immunoglobulins are all in the gamma region. In myeloma and some other disease states, a very high concentration of one particular immunoglobulin will show up as a monoclonal band.
Camelids (see camel) are unique among all other mammals in that they have fully functional immunoglobulins which consist of two heavy chains, but lacking the light chains usually paired with each heavy chain (they also have classical four-chain antibodies). The functional role of this separate repertoire is unknown as yet. Apart from providing insight into immunglobulin structure and antigen recognition in absence of light chain CDR's, these unusual antibodies can also be exploited to generate antibody fragments smaller yet than scFv's, but also much more stable.
Function
The antibodies have two primary functions:
- they bind antigens -- see below
- they combine with different immunoglobulin receptors specific for them and exert effector functions. These receptors are isotype-specific, which gives a great flexibility to the immune system, because different situations require only certain immune mechanisms to respond to antigens.
Affinity vs Avidity
- Affinity is the binding strength of the antibody to the antigen.
- Avidity is the number of antigen binding sites.
The humoral immune response
When a macrophage ingests a pathogen, it attaches parts of the pathogen's proteins to a class II MHC protein. This complex is moved to the outside of the cell membrane, where it can be recognized by a T lymphocyte, which compares it to similar structures on the cell membrane of a B lymphocyte. If it finds a matching pair, the T lymphocyte activates the B lymphocyte, which can produce antibodies only against the structure it presents on its surface.Antibodies exist freely in the bloodstream or bound to cell membranes. They are part of the humoral immune system. Antibodies exist in clonal lines that are specific to only one antigen, e.g., a virus hull protein. In binding to such antigens, they can cause agglutination and precipitation of antibody-antigen products primed for phagocytosis by macrophages and other cells, block viral receptors, and stimulate other immune responses, such as the complement pathway.
Antibodies that recognize viruses can block these directly by their sheer size. The virus will be unable to dock to a cell and infect it, hindered by the antibody. They can also agglutinate them so the phagocytes can capture them. Antibodies that recognize bacteria mark them for ingestion by phagocytes, a process called opsonization. Together with the plasma component complement, antibodies can kill bacteria directly. They neutralize toxins by binding with them.
It is important to note that antibodies cannot attack pathogens within cells, and certain viruses "hide" inside cells (as part of the lysogenic cycle) for long periods of time to avoid them (such as HIV and HBV). This is the reason for the chronic nature of many minor skin diseases (such as cold sores); any given outbreak is quickly suppressed by the immune system, but the infection is never truly eradicated because some cells retain viruses that will resume the apparent symptoms later.
Practical Applications
Medical Applications
Detection of particular antibodies is a very common form of medical diagnostics. Serology depends on these methods. Autoimmune disorders can often be traced to antibodies that bind the body's own epitopes; many can be detected through blood tests. Antibodies directed against RBC surface antigens in immune mediated hemolytic anemia can be detected with the Coombs test. The Coombs test is also used for antibody screening in blood transfusion preparation and also for antibody screening in antenatal women.
"Designed" monoclonal antibody therapy is already being employed in a number of diseases (including rheumatoid arthritis) and in some forms of cancer. Presently, many antibody-related therapies are undergoing extensive clinical trials for use in practice.
RHOGAM antibodies
RHOGAM antibodies are used in Rhesus-negative mothers who have a Rhesus-positive fetus. The Rhesus factor (a.k.a. D antigen) is an antigen found in blood. It is the second most important thing to consider in a blood transfusion, next to blood type. People that are Rh+ have this antigen on their blood cells. People that are Rh- don't have this antigen on their blood cells.
When the mother is Rh-, her body has B cells primed to produce antibodies that are specific to the Rh antigen, which essentially means that if the Rhesus antigen is injected into her blood, in about a week, all the antigen will be trashed. This is a problem for babies that are Rhesus-positive, because if childbirth has complications, and blood from the baby enters the mother, then the mother will treat the baby's blood cells as foreign enemies with Rhesus antigens sticking out, and will counter the invader with antibodies. The next time that the mother that has a baby with Rhesus-positive blood, the mother will actively attack the fetus, causing a serious condition where the baby will have massive red blood cell destruction, known as hemolysis. However, this can be prevented by an injection of RHOGAM antibodies.
RHOGAM antibodies are specific to Rh, i.e. that they were built to connect to the Rhesus antigen. They are used normally every time that a Rhesus-negative mother has a fetus that is Rhesus-positive. They will destroy the antigen before it can stimulate the mother's B cells to make anti-Rh antibodies. Therefore, her humoral immune system will never have to make anti-Rh antibodies, and will not attack the baby's Rhesus antigen.
Biochemical Applications
In biochemistry, antibodies are used for immunological identification of proteins using Western blot, ELISPOT, ELISA and flow cytometry. In these assays, antibodies are used to detect proteins such as cytokines, receptors or other antibodies. For example, a titer for Epstein-Barr virus or Lyme disease will look for if your body has produced antibodies specific to those antigens in your blood. If you don't have those antibodies, it is either you've never had the infection (so your body never had to produce them), or you had the infection a very long time ago, and your antibodies have naturally decayed.
Antibodies are also used to separate proteins (and anything bound to them) from the other molecules in a cell lysate.
These purified antibodies are often produced by injecting the antigen into a small mammal, such as a mouse or rabbit. Sometimes, in order to obtain large quantity of antibodies, goats, sheep, or horses are used. Blood isolated from these animals contains polyclonal antibodies -- multiple antibodies that stick to the same antigen. The serum (=blood from which blood-clotting proteins and red-blood cells were removed), also known as the antiserum, because it now contains the desired antibodies, is commonly purified with Protein A/G purification or antigen affinity chromatography. If the lymphocytes that produce the antibodies can be isolated and immortalized, then a monoclonal antibody can be obtained.
Antibodies are also widely used in immunohistochemical staining.
See also
References
External links
Antibody databases and protocols
- [Antibody] Protocols, news, antibody biology, and suppliers.
- [Antibody Search & Antibody Staining Protocols]
- [Antibody Staining Protocol Database]
- [Lymphomation: Immunoglobulins]
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| Humoral immune system | Cellular immune system | Immunological tolerance | Lymphatic system | White blood cells | Antibodies | Antigen (MHC) | Complement system | Inflammation |
| Immune system proteins - [http://encycl.opentopia.com/ edit] |
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| MAC complex | Perforin | Antibodies |
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