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Immunoglobulin G (IgG) is a type of antibody that represents 75% of the total antibodies found in human serum. An antibody is a Y-shaped protein that is released at the time of defence by the immune system to fight with foreign substances and microbes. They are glycoproteins that belong to the immunoglobulin superfamily.
Structure of IgG
The IgG antibody is a tetrameric quaternary structure that weighs about 150 KDa. It is a large globular protein that is made up of four peptide chains: two identical heavy chains, gamma (𝞬) and two identical lighter chains. The heavy chain weighs about 50 KDa each and the light chain 25 KDa each. The heavy chains are interconnected to each other and to a light chain by disulfide bonds. The tetramer formed has identical halves which when put together form a Y-shape. Both the forks of the Y-shaped antibody have an identical antigen binding site.
Structurally, an antibody can be divided into two antigen binding fragments called Fab that have the following domains: VL, VH, CL, and CH1 and an Fc region that forms the trunk of the Y-shaped antibody. The Fab domain has a variable antigen binding site called the Fv region that differs from antibody to antibody. The Fc region on the other hand is composed of constant domains of heavy chains.
Function of IgG
Antibodies such as IgG, IgM, IgE, IgA and IgD take part in humoral immunity. IgG is majorly found in extracellular fluid and blood. It binds to several pathogens such as viruses, bacteria and fungi and protects us from infection. IgG fights infections by the following mechanisms:
- The IgG molecule binds with the pathogen causing their immobilisation; the antibody-coated pathogen is easy to recognise and ingested by the phagocytic immune cells.
- IgG activates the classical complement pathway, releasing immune proteins that eliminate pathogens.
- It also binds to and neutralises toxins.
- It also helps in antibody-dependent cell-mediated cytotoxicity (ADCC) and intracellular antibody mediated proteolysis.
- IgG also plays a role in type I and II hypersensitivity reactions.
In immune responses, the IgG is released as a small molecule so that it can easily permeate through the tissues. It is unique in having receptors that allow it to pass through the placenta and thus provide protection to the foetus in utero. Along with IgA in breast milk, IgG provides humoral immunity to the infant before his or her own immunity develops. Thus, a child has (for the first six months) the same antibodies as his mother and can fight infections that his mother has encountered in her life. IgGs are also involved in the prevention of allergic reactions.
Refer: Difference Between Humoral And Cell Mediated Immunity
Subclasses of IgG
The IgG antibodies are divided into four subclasses:
|
Name |
Percentage |
Affinity to Fc receptor on phagocytic cells |
Crosses placenta easily |
Half life (days) |
|
IgG1 |
66% |
High affinity |
yes |
21 |
|
IgG2 |
23% |
Extremely low affinity |
no |
21 |
|
IgG3 |
7% |
High affinity |
yes |
7 |
|
IgG4 |
4% |
Intermediate affinity |
yes |
21 |
Clinical Significance
- Measure of IgG levels in one’s body can be indicative of a person’s immunity status to certain pathogens causing rubella, measles, mumps, Hepatitis B and varicella.
- Autoimmune hepatitis when presented with certain other symptoms can be diagnosed by checking IgG antibody levels.
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