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antibody,protein produced by the immune system (see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
..... Click the link for more information. ) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxinstoxin,
poison produced by living organisms. Toxins are classified as either exotoxins or endotoxins. Exotoxins are a diverse group of soluble proteins released into the surrounding tissue by living bacterial cells. Exotoxins have specific reaction sites in the host; e.g.
..... Click the link for more information. , viruses, or other cells or proteins. Such antigens are capable of inflicting damage by chemically combining with natural substances in the body and disrupting the body's processes. The body contains hundreds of thousands of different white blood cells called B lymphocytes, each capable of producing one type of antibody and each bearing sites on its membrane that will bind with a specific antigen. When such a binding occurs, it triggers the B lymphocyte to reproduce itself, forming a cloneclone,
group of organisms, all of which are descended from a single individual through asexual reproduction, as in a pure cell culture of bacteria. Except for changes in the hereditary material that come about by mutation, all members of a clone are genetically identical.
..... Click the link for more information. that manufactures vast amounts of its antibody.
The antibody molecule is composed of four polypeptide chains (see peptidepeptide,
organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
..... Click the link for more information. )—two identical light chains and two identical heavy chains—joined by disulfide bridges. The light chains have a variable portion that is different in each type of antibody and is the active portion of the molecule that binds with the specific antigen. Antibodies combine with some antigens, such as bacterial toxins, and neutralize their effect; they remove other substances from circulation in body fluids; they bind certain antigens together, a process known as agglutination; and they activate complement, blood serum proteins that cause the destruction of invading cells.
See also monoclonal antibodymonoclonal antibody,
an antibody that is mass produced in the laboratory from a single clone and that recognizes only one antigen. Monoclonal antibodies are typically made by fusing a normally short-lived, antibody-producing B cell (see immunity) to a fast-growing cell, such as
..... Click the link for more information. .
A protein found principally in blood serum and characterized by a specific reactivity with the corresponding antigen. Antibodies are important in resistance against disease, in allergy, and in blood transfusions, and can be utilized in laboratory tests for the detection of antigens or the estimation of immune status.
Antibodies are normally absent at birth unless derived passively from the mother through the placenta or colostrum. In time, certain antibodies appear in response to environmental antigens. Antibodies are also induced by artificial immunization with vaccines or following natural infections. The resulting antibody level declines over a period of months, but rapidly increases following renewed contact with specific antigen, even after a lapse of years. This is known as an anamnestic or booster response. See Allergy, Blood groups, Hypersensitivity, Isoantigen, Vaccination
Antibody reactivity results in precipitation of soluble antigens, agglutination of particulate antigens, increased phagocytosis of bacteria, neutralization of toxins, and dissolution of bacterial or other cells specifically sensitive to their action; the antibodies so revealed are termed precipitins, agglutinins, opsonins, antitoxins, and lysins. One antibody may give many such reactions, depending on conditions, so these classifications are not unique or exclusive.
Three principal groups (IgG, IgM, IgA) and two minor groups (IgD, IgE) of antibodies are recognized. These all form part of the wider classification of immunoglobulins. Antibody diversity is generated by amino acid substitutions that result in unique antigen-binding structures. See Cellular Immunology, Immunoglobulin
The development of the technology for producing monoclonal antibodies, which can bind to specific sites on target antigens, revolutionized the uses of antibodies in biology and medicine. Unfortunately, almost all monoclonal antibodies originate in mice, and the murine immunoglobulin serves as an antigen, frequently acting immunogenic in human recipients. See Antigen, Monoclonal antibodies