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. IgG molecules are synthesised and secreted by plasma B cells.

Functions

IgG antibodies are predominately involved in the secondary antibody response, (the main antibody involved in primary response is IgM) which occurs approximately one month following antigen recognition, thus the presence of specific IgG generally corresponds to maturation of the antibody response. Pro-inflammatory cytokines particularly IL-4 and IL-2, have a crucial role in activation of the IgG antibody response.

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.

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

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. The two heavy chains are linked to each other and to a light chain each by disulphide 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 receptors 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.

See also

• Antibody
• IgM, IgA, IgD, IgE (other heavy chain classes)
• B cell, Plasma B cell
• epitope

External links

• Janeway Immunobiology - The structure of a typical antibody (IgG)
• A booklet with everything you wanted to know about IgG subclasses

References