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Two types of IgA have been identified. They are serum IgA and secretory IgA (sIgA). In sIgA two IgA molecules are linked by a polypeptide called the secretory piece and by a J chain. Secretory IgA is present in nonvascular fluids, such as saliva, bile, synovial fluid, and intestinal and respiratory tract secretions. Both IgA types are known to have antiviral properties; their production is stimulated by oral vaccines and aerosol immunizations.
IgD is found in trace quantities in the serum (about 3 mg/dl). It serves as a B lymphocyte surface receptor.
IgE is called the reaginic antibody and is generally present in increased levels in persons with allergy. Its normal mean serum concentration is 0.03 mg/dl. When IgE attaches itself to cells within the body, such as those of the mucous membrane or skin, the cells become sensitized to allergens, causing them to release histamine and histamine-like substances when they come in contact with the allergen. Such allergic reactions as hives, hay fever, asthma, and anaphylactic shock are manifestations of IgE-mediated reactions.
IgG is the most abundant of the five classes of immunoglobulins. Its normal mean serum concentration is 1240 mg/dl. It is the major antibody in the secondary humoral response of immunity, serves to activate the complement system, and is frequently involved in opsonization. IgG is the only immunoglobulin that can cross the placental barrier.
IgM is principally concerned with the primary antibody response, appearing soon after initial invasion by an antigen and capable of destroying the antigen when it is first introduced. Its normal mean serum concentration is 120 mg/dl. Like IgG, IgM activates the complement system and together these two classes of immunoglobulins serve as specific antitoxins against the toxins of diphtheria, tetanus, botulism, and anthrax microorganisms, and snake venoms.
When split by papain, IgG yields three pieces: the Fc piece, consisting of the C-terminal portion of the H chains, with no antibody activity but capable of fixing complement, and crystallizable; and two identical Fab pieces, carrying the antigen-binding sites and each consisting of an L chain bound to the remainder of an H chain.
Antibodies are immunoglobulins, and all immunoglobulins probably function as antibodies. However, immunoglobulin refers not only to the usual antibodies, but also to a great number of pathologic proteins classified as myeloma proteins, which appear in multiple myeloma along with Bence Jones proteins, myeloma globulins, and immunoglobulin fragments.
From the amino acid sequences of Bence Jones proteins, it is known that all L chains are divided into a region of variable sequence (VL) and one of constant sequence (CL), each comprising about half the length of the L chain. The constant regions of all human L chains of the same type (κ or λ) are identical except for a single amino acid substitution, under genetic controls. H chains are similarly divided, although the VH region, although similar in length to the VL region, is only one third or one fourth the length of the CH region. Binding sites are a combination of VL and VH protein regions. The large number of possible combinations of L and H chains make up the "libraries" of antibodies of each individual.
immunoglobulin/im·mu·no·glob·u·lin/ (-glob´ūl-in) a protein of animal origin with known antibody activity, synthesized by lymphocytes and plasma cells and found in serum and in other body fluids and tissues; abbreviated Ig. There are five distinct classes based on structural and antigenic properties: IgA, IgD, IgE, IgG, and IgM.
immunoglobulinA highly-specific molecule of the immune system, produced by mature B cells in response to an antigen Structure 2 identical light-L, 2 identical heavy–H chains; the L and H chains have constant and variable regions, the variable regions are critical for antigen recognition and binding; immunoglobulin production requires prior rearrangement of the variable, diversity and joining gene segments, that form part of a potential repertoire of 1010-1012 antibodies that may be encoded in response to a molecule's surface binding site or epitope Types Idiotype–evoked by a particular epitope; isotype–Ig subtype–IgG, IgA, IgM, IgD, IgE that all normal persons have; allotype–a subtype shared by population groups–eg, with racial differences. See Hinge, Polyclonal immunoglobulin, Protein electrophoresis, Sporidin-G™ Bovine immunoglobulin.
immunoglobulina protein (such as gamma globulin) made in B-CELLS that possesses ANTIBODY activity and is made up of four POLYPEPTIDE CHAINS, two identical light (L) chains and two identical heavy (H) chains joined by DISULPHIDE BRIDGES to form a Y-shaped macromolecule. The H chains are connected by two or more disulphide bridges at the hinge region of the Y. One 1 chain runs beside each of the two limbs of the Y, attached by a disulphide bridge. There are five main classes of human immunoglobins, differentiated principally by their heavy chains:
|type||heavy chain||m.w. (x 1000)|
Each heavy and light chain consists of a CONSTANT REGION (C region or domain) of amino acids that is virtually the same in all antibodies of that class, and a VARIABLE REGION (V region or domain) containing a sequence of amino acids that makes the antibody specific to a particular ANTIGEN.
Only a few of these amino acids actually contact the antigen and this contact region is called the complementarity determining region (CDR). For the genetics of immunoglobulin production see C-GENE, D-GENE, J-GENE and V-GENE.