Immunoglobulin Classes
المؤلف:
Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia
المصدر:
Jawetz, Melnick, & Adelberg’s Medical Microbiology
الجزء والصفحة:
28e , p137-138
2025-07-19
614
A. IgG
IgG is the major class of immunoglobulin present in the serum. The IgG molecule consists of two L chains and two H chains (H2L2) (Figure 1). There are four subclasses of IgG: IgG1, IgG2, IgG3, and IgG4. Each subtype contains a distinct but related H chain and each differs somewhat regarding their biological activities. IgG1 represents 65% of the total IgG. IgG2 is directed against polysaccharide antigens and may be an important host defense against encapsulated bacteria. IgG3 is an effective activator of complement due to its rigid hinge region, whereas IgG4 does not activate complement due to its compact structure.
Fig1. Schematic representation of an IgG molecule, indicating the location of the constant and the variable regions on the light and heavy chains. Fab fragment is fragment antigen binding, Fc fragment is fragment crystallizable.
IgG is the only immunoglobulin class to cross the placenta and therefore is the most abundant immunoglobulin in newborns. Isotype-specific transport of IgG across the placenta occurs with preference for IgG1 and IgG3 subclasses. IgG also mediates opsonization of antigen through binding of antigen-antibody complexes to Fc receptors on macro phages and other cells.
B. IgM
The first immunoglobulin produced in response to an anti gen is IgM. IgM is secreted as a pentamer and is composed of five H2L2 units (similar to one IgG unit) and one molecule of a J chain (Figure 2). The pentamer (MW 900,000) has a total of 10 identical antigen-binding sites and thus a valence of 10. It is the most efficient immunoglobulin in agglutination, complement fixation, and other antigen–antibody reactions and is important also in defense against bacteria and viruses. Because its interaction with antigen can involve all 10 binding sites, it has the highest binding capacity and cross-linking of all the immunoglobulins. Evaluating the presence of serum IgM may be useful in the diagnosis of certain infectious diseases. For example, IgM does not cross the placenta and the presence of the IgM antibody in the fetus or newborn provides evidence of intrauterine infection.
Fig2. Schematic diagram of the pentameric structure of human IgM. The IgM monomers are connected to each other and the J chain by disulphide bonds.
C. IgA
IgA is the major immunoglobulin responsible for mucosal immunity. The levels of IgA in the serum are low, consisting of only 10–15% of total serum immunoglobulins present. In contrast, IgA is the predominate class of immunoglobulin found in extravascular secretions. Thus, plasma cells located in glands and mucous membranes mainly produce IgA. Therefore, IgA is found in secretions such as milk, saliva, and tears, and in other secretions of the respiratory, intestinal, and genital tracts. These locations place IgA in contact with the external environment and therefore can be the first line of defense against bacteria and viruses.
The properties of the IgA molecule are different depending on where IgA is located. In serum, IgA is secreted as a monomer resembling IgG. In mucous secretions, IgA is a dimer and is referred to as secretory IgA. This secretory IgA consists of two monomers that contain two additional poly peptides: the J chain that stabilizes the molecule and a secretory component that is incorporated into the secretory IgA when it is transported through an epithelial cell. There are at least two IgA subclasses: IgA1 and IgA2. Some bacteria (eg, Neisseria spp.) can destroy IgA1 by producing a protease and can thus overcome antibody-mediated resistance on mucosal surfaces.
D. IgE
The IgE immunoglobulin is present in very low quantities in the serum. The Fc region of IgE binds to its high-affinity receptor on the surface of mast cells, basophils, and eosinophils. This bound IgE acts as a receptor for the specific antigen that stimulated its production and the resulting antigen antibody complex triggers allergic responses of the immediate (anaphylactic) type through the release of inflammatory mediators such as histamine.
E. IgD
Serum IgD is present only in trace amounts. However, IgD is the major surface bound immunoglobulin on mature B lymphocytes that have not yet encountered antigen. These B cells contain IgD and IgM at a ratio of 3 to 1. At the present time, the function of IgD is unclear.
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