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Drawing of an IgG antibody - the two heavy chains are seen in red, the two light chains are yellow.

Immunoglobulin G (IgG) is a monomeric immunoglobulin, built of two heavy chains γ and two light chains. Each IgG has two antigen binding sites. It is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids, constituting 75% of serum immunoglobulins in humans.[1] IgG molecules are synthesised and secreted by plasma B cells.

Contents

Functions

IgG antibodies are predominately 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.[2]

This 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.

It 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.

Structure

Immunoglobulin

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.

Subclasses

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
IgG3 7% yes highest high affinity
IgG4 4% yes no intermediate 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.

Receptors

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. [3]

See also

External links

References

  1. ^ Junqueira, Luiz C.; Jose Carneiro (2003). Basic Histology. McGraw-Hill. ISBN 0838505902.  
  2. ^ 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)
  3. ^ Male, D., Brostoff, J., Roth, DB., & Roitt, I. 2006. Immunology, 7th Edition. Mosby Publishing.
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