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Immunoglobulin G (IgG) are antibody molecules. Each IgG is composed of four peptide chains -- two heavy chains γ and two light chains. Each IgG has two antigen binding sites. Other Immunoglobulins may be described in terms of polymers with the IgG structure considered the monomer.
IgG is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids, constituting 75% of serum immunoglobulins in humans. IgG molecules are synthesized and secreted by plasma B cells.
IgG antibodies are predominantly involved in the secondary immune response (the main antibody involved in primary response is IgM). The presence of specific IgG generally corresponds to maturation of the antibody response.
IgG is the only isotype that can pass through the human placenta, thereby providing protection to the fetus in utero. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the neonate with humoral immunity before its own immune system develops. Colostrum contains a high percentage of IgG, especially in bovine colostrum.
IgG can bind to many kinds of pathogens, for example viruses, bacteria, and fungi, and protects the body against them by agglutination and immobilization, complement activation (classical pathway), opsonization for phagocytosis and neutralization of their toxins. It also plays an important role in Antibody-dependent cell-mediated cytotoxicity(ADCC).
IgG is also associated with Type II and Type III Hypersensitivity.
IgG antibodies are large molecules of about 150 kDa composed of 4 peptide chains. It contains 2 identical heavy chains of about 50 kDa and 2 identical light chains of about 25 kDa, thus tetrameric quaternary structure. The two heavy chains are linked to each other and to a light chain each by disulfide bonds. The resulting tetramer has two identical halves which together form the Y-like shape. Each end of the fork contains an identical antigen binding site. The Fc regions of IgGs bear a highly conserved N-glycosylation site. The N-glycans attached to this site are predominantly core-fucosylated diantennary structures of the complex type. Additionally, small amounts of these N-glycans also bear bisecting GlcNAc and α-2,6 linked sialic acids residues.
There are four IgG subclasses (IgG1, 2, 3 and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).
|Name||Percent||Crosses placenta easily||Complement activator||Binds to Fc receptor on phagocytic cells|
|IgG1||66%||yes||second highest||high affinity|
|IgG2||23%||no||third highest||extremely low affinity|
Note: IgG affinity to Fc receptors on phagocytic cells is specific to individual species from which the antibody comes as well as the class. The structure of the hinge regions gives each of the 4 IgG classes their unique biological profile. Even though there is about 95% similarity between their Fc regions, the structure of the hinge regions are relatively different.
In humans, the three receptors for IgG are:
FcγRI (CD64) – 72kDa in size. Expressed on cells of mononuclear phagocyte lineage.
FcγRII (CD32) – 40kDa in size. Has 2 forms, alpha (with an ITAM receptor motif) and beta (with an ITIM receptor motif).
FcγRIII (CD16) – 50-80kDa in size. Has 2 forms, alpha (a transmembrane protein) and beta (expressed on neutrophils).
Glycosylation is essential for IgG binding to its receptors, regardless of its class. 
- IgM, IgA, IgD, IgE (other heavy chain classes)
- B cell, Plasma B cell
- Janeway Immunobiology - The structure of a typical antibody (IgG)
- A booklet with everything you wanted to know about IgG subclasses
- ↑ Junqueira, Luiz C.; Jose Carneiro (2003). Basic Histology, McGraw-Hill.
- ↑ Meulenbroek, A.J.; Zeijlemaker, W.P. (1996). Human IgG Subclasses: Useful diagnostic markers for immunocompetence. Published by Sanquin formerly CLB (Centraal Laboratorium van de Bloedtransfusiedienst)
- ↑ Stadlmann J, Pabst M, Kolarich D, Kunert R, Altmann F. (2008) Analysis of immunoglobulin glycosylation by LC-ESI-MS of glycopeptides and oligosaccharides. Proteomics. 2008 Jul;8(14):2858-71
- ↑ Male, D., Brostoff, J., Roth, DB., & Roitt, I. 2006. Immunology, 7th Edition. Mosby Publishing.
Immune system / Immunology / Psychoneuroimmunology
|Systems||Adaptive immune system vs. Innate immune system • Humoral immune system vs. Cellular immune system • Complement system (Anaphylatoxins)|
|Antibodies and antigens||Antibody (Monoclonal antibodies, Polyclonal antibodies, Autoantibody) • Allotype • Isotype • Idiotype • Antigen (Superantigen)|
|Immune cells||White blood cells (T cell, B cell, NK cell, Mast cell, Basophil, Eosinophil) • Phagocyte (Neutrophil, Macrophage, Dendritic cell) • Antigen-presenting cell • Reticuloendothelial system|
|Immunity vs. tolerance||Immunity • Autoimmunity • Allergy • Tolerance (Central) • Immunodeficiency|
|Immunogenetics||Somatic hypermutation • V(D)J recombination • Immunoglobulin class switching • MHC / HLA|
|Other||Cytokines • Inflammation • Opsonin|
Immune system proteins
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