Wiley; Monash University (Australia)
Immunoglobulins (antibodies) are the glycoproteins involved in the immune response. As well as the adaptive immunity, these molecules are the evolutionary innovationof vertebrates. These proteins can recognize and bind to hazardous agents (bacteria, fungi, viruses, protozoa, worms, the body’stumor cells, toxins), triggering a cascade of other processes aimed at destroying them.The organism can generate a specific antibody to any antigen it encounters: about 10 billion unique immunoglobulin species can be synthesized in a period of the human life (1).
However only three genetic loci are responsible for the immunoglobulin synthesis. The huge diversity of antibodies is possible because of the specific mechanisms of gene rearrangements and introduction of point mutations (2, 3). Immunoglobulins are found both onthe surfaces of the B-lymphocytes where they are synthesized and in blood, tissue fluid, mucous membranes, saliva, tears, or breast milk. Theaverage human produces about 2-3 grams of antibodies per day. Two-thirds of this amount is produced in digestive, respiratory, and urogenital tracts (4).
Immunoglobulins consist of two identical heavy and two identical light polypeptide chains, connected by disulfide bonds. Heavy chains are glycosylated. There are five types of heavy chains (α-, γ-, δ-, ε- and μ-chains) and two types of light chains (κ-chain and λ-chain). Both heavy and light chains are divided into domains (4 and 2, respectively). One of the domains is variable and the other(s) are constant. The part of the molecule that is bound to the antigen is calledthe Fragment Antigen Binding (FAB), There are two FABs in each monomeric antibody. These fragments are formed by the whole light chain and two domains (one variable and one constant) of the heavy chain. Thus all variable segments are part of the antigen binding regions. A constant part of the immunoglobulin (FC — Fragment crystallizable) is formed by the heavy chains constant domains. FABs are flexibly attached to the FC atthe hinge region. This providesthe molecule its Y- or T-shape.There are 5 classes of antibodies that differ in function, structure, size and glycosylation pattern: IgG, IgD, IgE, IgA and IgM.
Molecules of the IgG, IgD, IgE and the serum IgA work as monomers, but the secreted IgA (found in saliva) is a dimer and the IgM molecules form pentamers (2, 4). IgGs, the most common human immunoglobulins, represent 75% of all plasma immunoglobulins and have several functions including triggering the complement system (the proteolytic plasma enzyme cascade involved in antigen degradation in the blood).
The model presented provides a view of the common IgG1 protein. Light chains are shown in red, heavy chains are in gray, disulphide bridges linking the chains are in yellow, and the oligosaccharide is in dark gray.
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