File Name: structure and function of immunoglobulins .zip
Antibody , also called immunoglobulin , a protective protein produced by the immune system in response to the presence of a foreign substance, called an antigen. Antibodies recognize and latch onto antigens in order to remove them from the body. A wide range of substances are regarded by the body as antigens, including disease-causing organisms and toxic materials such as insect venom.
Immunoglobulin Structure and Classes
Antibody , also called immunoglobulin , a protective protein produced by the immune system in response to the presence of a foreign substance, called an antigen.
Antibodies recognize and latch onto antigens in order to remove them from the body. A wide range of substances are regarded by the body as antigens, including disease-causing organisms and toxic materials such as insect venom. When an alien substance enters the body, the immune system is able to recognize it as foreign because molecules on the surface of the antigen differ from those found in the body.
To eliminate the invader, the immune system calls on a number of mechanisms, including one of the most important—antibody production. Antibodies are produced by specialized white blood cells called B lymphocytes or B cells. When an antigen binds to the B-cell surface, it stimulates the B cell to divide and mature into a group of identical cells called a clone. The mature B cells, called plasma cells, secrete millions of antibodies into the bloodstream and lymphatic system.
As antibodies circulate, they attack and neutralize antigens that are identical to the one that triggered the immune response. Antibodies attack antigens by binding to them. The binding of an antibody to a toxin , for example, can neutralize the poison simply by changing its chemical composition; such antibodies are called antitoxins.
By attaching themselves to some invading microbes, other antibodies can render such microorganisms immobile or prevent them from penetrating body cells. In other cases the antibody-coated antigen is subject to a chemical chain reaction with complement , which is a series of proteins found in the blood. The complement reaction either can trigger the lysis bursting of the invading microbe or can attract microbe-killing scavenger cells that ingest, or phagocytose , the invader.
Once begun, antibody production continues for several days until all antigen molecules are removed. Antibodies remain in circulation for several months, providing extended immunity against that particular antigen.
B cells and antibodies together provide one of the most important functions of immunity, which is to recognize an invading antigen and to produce a tremendous number of protective proteins that scour the body to remove all traces of that antigen. Collectively B cells recognize an almost limitless number of antigens; however, individually each B cell can bind to only one type of antigen.
B cells distinguish antigens through proteins, called antigen receptors , found on their surfaces. An antigen receptor is basically an antibody protein that is not secreted but is anchored to the B-cell membrane. All antigen receptors found on a particular B cell are identical, but receptors located on other B cells differ. Although their general structure is similar, the variation lies in the area that interacts with the antigen—the antigen-binding, or antibody-combining, site.
This structural variation among antigen-binding sites allows different B cells to recognize different antigens. Binding between the receptor and epitope occurs only if their structures are complementary. If they are, epitope and receptor fit together like two pieces of a puzzle, an event that is necessary to activate B-cell production of antibodies.
Each antibody molecule is essentially identical to the antigen receptor of the B cell that produced it. The basic structure of these proteins consists of two pairs of polypeptide chains lengths of amino acids linked by peptide bonds that form a flexible Y shape.
The stem of the Y consists of one end of each of two identical heavy chains, while each arm is composed of the remaining portion of a heavy chain plus a smaller protein called the light chain.
The two light chains also are identical. Within particular classes of antibodies the stem and the bottom of the arms are fairly similar and thus are called the constant region.
The tips of the arms, however, are highly variable in sequence. It is these tips that bind antigen. Thus each antibody has two identical antigen-binding sites, one at the end of each arm, and the antigen-binding sites vary greatly among antibodies.
Antibodies are grouped into five classes according to their constant region. The classes of antibody differ not only in their constant region but also in activity. For example, IgG, the most common antibody, is present mostly in the blood and tissue fluids, while IgA is found in the mucous membranes lining the respiratory and gastrointestinal tracts. Preformed antibodies, which are derived from the blood serum of previously infected people or animals, are often administered in an antiserum to another person in order to provide immediate, passive immunization against fast-acting toxins or microbes, such as those in snakebites or tetanus infections.
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The four-chain structure of an antibody, or immunoglobulin, molecule. The basic unit is composed of two identical light L chains and two identical heavy H chains, which are held together by disulfide bonds to form a flexible Y shape. Each chain is composed of a variable V region and a constant C region. Britannica Quiz. Medical Terms and Pioneers Quiz. Who discovered the major blood groups? What causes the blood disease thalassemia? Test what you know about medical science by taking this quiz.
Transmission electron micrograph of a human B cell, or B lymphocyte. Phagocytic cells destroy viral and bacterial antigens by eating them, while B cells produce antibodies that bind to and inactivate antigens. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Variable V and constant C domains within the light L and heavy H chains of an antibody, or immunoglobulin, molecule.
Learn More in these related Britannica articles:. The antigen receptors on B lymphocytes are identical to the binding sites of antibodies that these lymphocytes manufacture once stimulated, except that the receptor molecules have an extra tail that penetrates the cell membrane and anchors them to the cell surface. Thus, a description…. Antibodies, proteins that combat foreign substances in the body, are associated with the globulin fraction of the immune serum.
Antibodies can…. Immune serum globulin ISG , obtained from the plasma of a pool of healthy donors, contains a mixture of immunoglobulins, mainly IgG, with lesser amounts of IgM and IgA. It is used to provide passive immunity to a variety of diseases such as measles, hepatitis…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox! Email address. By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
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The produced antibodies bind to specific antigens express in external factors and cancer cells. The basic structure of all antibodies are same. An antibody is made up of a variable region and a constant region, and the region that changes to various structures depending on differences in antigens is called the variable region , and the region that has a constant structure is called the constant region. Each heavy and light chain in an immunoglobulin molecule contains an amino-terminal variable V region that consists of to amino acids and differ from one antibody to another. The remainder of each chain in the molecule — the constant C region exhibits limited variation that defines the two light chain subtypes and the five heavy chains subclasses. The amino terminal portions, corresponding to the V regions, bind to antigen; effector functions are mediated by the carboxy-terminal domains.
Where does the name g -globulin come from? Antibodies are secreted and they also exist as the B-cell receptor BCR. The B-cell receptor also has other proteins associated with it:. Antibodies have two fundamental characteristics:. Specificity — the ability to bind to epitopes.
Antibody- Structure, Classes and Functions
This page introduces the nomenclature and criteria used to describe the structure, classes and functional types of immunoglobulins. Antibody or immunoglobulin molecules are glycoproteins composed of one or more units, each containing four polypeptide chains: two identical heavy chains H and two identical light chains L. The amino terminal ends of the polypeptide chains show considerable variation in amino acid composition and are referred to as the variable V regions to distinguish them from the relatively constant C regions. Heavy and light chains are held together by a combination of non-covalent interactions and covalent interchain disulfide bonds, forming a bilaterally symmetric structure.