immunoglobulin

n. (Abbr. Ig)
Any of a group of large glycoproteins that are secreted by plasma cells and that function as antibodies in the immune response by binding with specific antigens. There are five classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.

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 also 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 also Anaphylaxis; Antibody; Antigen; Antigen-antibody reaction; Hypersensitivity; Immunology; Protein.

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(im'yōōnō-glob'yōōlinz)
n.pl
Ig

Serum proteins (g globulins) synthesized by plasma cells that act as antibodies and are important in the body’s defense mechanisms against infection. Main classes are designated as IgG, IgA, and IgM.

A protein in the blood (e.g. gamma-globulin) that possess antibody activity. Immunoglobulin is an important part of the body's immune system. See also salivary immunoglobulin A.

A specialized class of serum proteins, which may occur naturally in serum, but are usually produced following exposure to an almost limitless (>10⁷) number of antigens. Called also antibody. Immunoglobulins combine only with the antigen (or one closely related to it) that elicited their production. Immunoglobulins are major components of what is called the humoral immune response system. They are synthesized by B lymphocytes and their derivative plasma cells, and are found in the serum and in other body fluids and tissues, including the urine, spinal fluid, lymph nodes and spleen. See also immunity.
Immunoglobulin molecules consist of two kinds of polypeptide chains: heavy chains (H-chains) and light chains (L-chains). There are five antigenically different kinds of H-chains, designated γ, μ, α, δ and ε, and this difference is the basis for the classification of immunoglobulins. Classes vary in their chemical structure and in the number of antigen-binding sites.
The five classes of immunoglobulins (Ig) are: IgG, IgM, IgA, IgD and IgE. Only IgG, IgM and IgA are found in all species of domestic animals.
IgA is present in low concentrations in the serum, but it is the major immunoglobulin of secretions and has a major first-line defense role in infections that enter via mucosal surfaces. 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 secreted and circulating IgA types are known to have antiviral properties; their production is preferentially stimulated by local administration of antigens such as oral and aerosol immunizations.
IgD is found in trace quantities in the serum in humans and chickens. It is found on the surface of B lymphocytes. Its function is uncertain.
IgE, once called reaginic antibody, is present in very low levels in serum and is generally present in increased levels in individuals with allergy. It has not been found in the chicken. IgE binds to Fc receptors on the surface of cells particularly mast cells and basophils, via the Fc part of the molecule. Following exposure to antigen (allergen), and its binding to the Fab of two adjacent IgE molecules, perturbations of the cell membrane are produced, leading to the release of vasoactive amines, particularly histamine and serotonin, which are the mediators of anaphylaxis and atopic reactions, including urticaria, asthma, hayfever and gastroenteritis. Allergic reactions such as urticaria, atopy and anaphylactic shock are examples of IgE-mediated reactions. It is recognized in humans and dogs that there is an inherited (familial) predisposition for certain individuals to produce IgE.
IgG is the most abundant of the five classes of immunoglobulins, representing about 80% of serum immunoglobulin protein. 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 crosses the placenta and is the major component of passive maternal antibody transfer via colostrum and yolk.
IgM is the first antibody produced in the primary immune response. It represents about 20% of serum antibodies. Like the IgG, IgM bound to antigen 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, and play a major role in defense against most infectious diseases.

Structure of an immunoglobulin molecule. By permission from Tizard IR, Veterinary Immunology. An Introduction, Saunders, 2001

• colostral i. — colostrum contains a high level of IgG for several days after parturition. Following ingestion, colostral IgG molecules are absorbed unchanged across the intestinal mucosa for the first 1 to 2 days in cattle, dog, pig and horse and for up to 4 days in the sheep and goat. IgA is also present in colostrum but is not translocated to the circulation of the suckling animal to any extent; it may provide some local gut immunity.
• i. deficiency — see immunodeficiency.
• i. genes — genes that code for the light and heavy chains of immunoglobulins.
• secretory i. — IgA.
• i. superfamily — immunoglobulins and a number of other proteins including T cell receptor, major histocompatibility molecules, T cell accessory proteins and some adhesion molecules that are structurally related to immunoglobulins.