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usually IgA but may be IgM linked to a secretory component and found in mucous secretions.
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.
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.
genes that code for the light and heavy chains of immunoglobulins.
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.