|
Background
|
The human immune system consists of two functional components classified as the innate system (the physical, biochemical and cellular barriers), and the adaptive immune system (including lymphocytes and immunoglobulins). Immunoglobulins are the key elements of the humoral immune response in vertebrate against parasitic invasion. The polypeptide chains of immunoglobulins composed of two identical heavy (H) chains and two identical light (L) chains linked together by inter-chain disulfide bonds. While the amino-terminal portions that exhibits highly variable amino-acid composition are involved in antigen binding, the C terminal constant parts are involved in complement binding, placental passage and binding to cell membranes. Based upon the variation of the constant region of the heavy chain, nine immunoglobulin heavy chain isotypes are found in humans: IgA (with subclasses IgA1 and IgA2), IgD, IgE, IgM, and IgG (with subclasses IgG1, IgG2, IgG3, and IgG4). IgG is the predominant immunoglobulin in the serum (about 12 mg/ml), which accounts for 75% of the total serum antibody of healthy individuals. IgG has a molecular weight of about 150 kDa. Four distinct subgroups of human IgG (IgG1, IgG2, IgG3, and IgG4) were first demonstrated in the 1960's by using polyclonal antisera prepared in animals immunized with human myeloma proteins. Quantitatively, the relative abundance of the four subclasses in ***** human serum follows IgG1 > IgG2 > IgG3 = IgG4, which accounts for 6.98, 3.8, 0.56, and 0.56 mg/ml respectively. IgA exists as a 160kd monomer in serum and as a 400kd dimer in secretions. Quantitatively, IgA is synthesized in amounts greater than IgG. However, due to its short half life in serum (6 days of IgA vs 21 days of IgG) and the lost of secretory form, the normal IgA serum level (2-3 mg/ml) ranked second after IgG, which accounts for 15% of the total antibody. There are two subclasses based on different heavy chains, IgA1 and IgA2. IgA1 is produced in bone marrow and makes up over 90% of the serum IgA. Secretory IgA is the predominant immunoglobulin present in gastrointestinal fluids, nasal secretions, saliva, tears and other mucous secretions of the body. IgM is the third most common serum immunoglobulin (about 1.5 mg/ml) which makes up about 10% serum antibody. IgM normally exists as a pentamer (about 900 kDa) and has a theoretical valence of 10. As a consequence of its pentameric structure, IgM is a good antigen agglutinating and complement fixing immunoglobulin. IgE exists as a 190 kDa monomer and is the least common serum immunoglobulins which accounts for 0.002% of the total serum antibodies. IgE is involved in allergic reactions. If an infectious agent succeeds in penetrating the IgA barrier, it comes up against the next line of defense, the IgE manned MALT (mucosa-associated lymphoid tissues) system. Contact with the allergen leads to the release of various pharmacological mediators that result in allergic symptoms. IgD is a 175kd molecule that resembles IgG in its monomeric form. IgD is found in low level in serum (0.03 mg/ml) with uncertain serological functions. IgD antibodies are found for the most part on the surfaces of B lymphocytes. It is thought that IgD and IgM function as mutually-interacting antigen receptors for control of B-cell activation and suppression. Hence, IgD may have an immunoregulatory function. The levels of different immunoglobulin subclasses follow a typical pattern in a healthy ethnic ***** and are normally within a certain percentile ranges. Upon different antigenic stimulation, an antibody response will behave differently in the distribution of the different subclasses in plasma, such as increase, diminish or even the deficiency of producing one of the specific immunoglobulin subclass. Over the last decades numerous reports have appeared on the distribution of the immunoglobulin subclasses produced during immune responses to bacterial, viral, and parasitic antigens, autoantigens, tumor antigens, and many parenterally administered substances such as hormones, drugs, and allergens. As a result, quantification of the immunoglobulin isotype level in a given serum sample will provide the useful information about the myeloma states as well as in monitoring intravenous immunoglobulin replacement, plasmaphoresis, and immunosuppression therapy.
|
