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see antibodyantibody,
protein produced by the immune system (see immunity) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxins, viruses, or other cells or proteins.
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; 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.
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; immunologyimmunology,
branch of medicine that studies the response of organisms to foreign substances, e.g., viruses, bacteria, and bacterial toxins (see immunity). Immunologists study the tissues and organs of the immune system (bone marrow, spleen, tonsils, thymus, lymphatic system),
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Any of the glycoproteins in the blood serum that are induced in response to invasion by foreign antigens and that protect the host by eradicating pathogens. Antibodies belong to this group of proteins. An antigen is any substance capable of inducing an immune response. Intact antigens are able to specifically interact with the induced immunoglobulins. Normally, the immune system operates in a state known as self-tolerance, and does not attack the host's own tissues, but occasionally the immune system targets host-specific antigens, resulting in autoimmune disease. See Autoimmunity

Immunoglobulins are composed of two identical heavy (H) and two identical light (L) polypeptide chains. Each H and L chain has an amino-terminal variable (V) region and a carboxyl-terminal constant (C) region. Although V regions from different antibodies exhibit considerable sequence variation, there is a large degree of sequence similarity among C regions of different antibodies. In the living animal, antibodies first bind to antigen at the antigen combining site and then, ideally, eliminate it as a threat to the host.

Immunoglobulins are heterogeneous with respect to charge, size, antigenicity, and function. There are three categories of antigenic determinants present on immunoglobulins: isotypes are found in all individuals, allotypes are found in some individuals, and idiotypes are associated with the amino-terminal variable region. Isotypic determinants are located on the carboxyl-terminal constant region and are used to group immunoglobulin H and L chains into isotypes or classes. In total, there are five human H-chain classes. IgM contains mu (μ) H chains, IgG contains gamma (γ) H chains, IgA contains alpha (α) H chains, IgD contains delta (δ) H chains, and IgE contains epsilon (ε) H chains. IgG has four subclasses, IgG1, IgG2, IgG3, and IgG4, while IgA has two subclasses, IgA1 and IgA2. There are two L-chain isotypes named kappa (κ) and lambda (λ). Kappa and lambda chains may be associated with H chains of any isotype, and a complete description of an immunoglobulin molecule requires identification of both H and L chains.

IgG is the most abundant immunoglobulin class in the serum. IgG isotypes are associated with complement fixation, opsonization (that is, rendering more susceptible to phagocytosis), fixation to macrophages, and membrane transport. Of the two IgA subclasses, IgA1 is the predominant subclass of IgA in human serum. IgA1 is the dominant subclass in all external secretions, including milk, saliva, tears, and bronchial fluids. The percentage of subclass IgA2 is higher in these fluids than in serum. IgM is the first immunoglobulin to appear during the primary immune response. IgD and IgE are present in minute amounts in normal human serum. No function has been clearly attributed to IgD. IgE is active against parasites and acts as a mediator of immediate hypersensitivity. See Anaphylaxis, Antibody, Antigen, Antigen-antibody reaction, Hypersensitivity, Immunology, Protein


Any of a set of serum glycoproteins which have the ability to bind other molecules with a high degree of specificity. Abbreviated Ig.
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