Types of IgG

Immunoglobulins are antibodies which are encoded for by large multigene families. Immunoglobulins are shaped by the process of somatic rearrangement, along with other genetic changes, during the initial development of B lymphocytes.

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Immunoglobulins are found inside all vertebrates and act as the initial detector of any foreign bodies that come into contact with the host organisms’ bodily systems, such as pathogenic bacteria, for example.

They also mediate the body’s response to a pathogenic infection by initiating inflammation and opsonizing foreign bodies for a more effective immune response.

IgG is the type of immunoglobulin most commonly found within human serum and can be broken down into four sub-classes: IgG₁, IgG₂, IgG₃, and IgG₄.

IgG’s make up 10-20% of human plasma and were discovered in the 1960’s after studies were performed on rabbit antisera against human IgG proteins.

Different subclasses of IgG have approximately 90% similarity with respect to their amino acid structures, but each subclass has unique antigen binding methods, complement activation, immune complex formations, half-life, and trigger their respective effector cells in different ways.

What does each IgG subclass do?

IgG₁ antibodies

IgG₁ antibodies are produced when an organism experiences an anaphylactic or allergic reaction to a substance, along with IgE.

The production of IgG₁ has mainly been observed to induce essential cardio-pulmonary responses to anaphylaxis, with these responses mainly being mediated by the ‘Fc’ region of the IgG₁ antibody.

IgG₂ antibodies

IgG₂ antibodies are usually found in relation with the host reaction to bacterial capsular polysaccharide antigens, with IgG₂ deficiency being associated with an increased susceptibility to certain bacterial infections.

During the course of infection, when certain released cytokines induce the expansion of B-cell clones within the human immune system, the pre-committed B-cells have been shown to induce the secretion of IgG₂ in vitro.

IgG₃ antibodies

IgG3 antibodies are mostly found to be associated with effector responses to foreign antigens. They are also produced when the human body becomes infected with a parasite.

For example, during a cross-sectional survey carried out in Western Africa in 1998, 178 individuals aged between 1-75 years were vaccinated against Plasmodium falciparum – a type of malaria – and their antibody responses assessed.

It was observed that in the individuals aged 10 years or above, the specific antibodies that were produced were mostly IgG₃.

It has therefore been hypothesized that the age- or exposure-related acquisition of IgG₃ antibodies might play a role in the development of clinically protective immunity to the Plasmodium parasite.

IgG₄ antibodies

The final immunoglobulin subclass IgG₄ is commonly associated with chronic exposure to an antigen, as in immunotherapy, and with filarial or helmintic infection. IgG₄ has also been associated with several autoimmune diseases, including asthma, pancreatitis, and certain allergies.

IgG₄ is therefore used as a biological marker for many diseases, as significantly higher levels are indicative of an immune response.  For example, higher than average volumes of IgG₄ in the blood and pancreatic tissue can facilitate the diagnosis of autoimmune pancreatitis.

The IgG₄ antibody has a similar physical structure and half-life to IgG₂ – with both of these IgG subclasses being seen in conjunction with IgE (immunoglobulin E), which activates mast cell degranulation, usually resulting in allergies and/or asthma.

Conclusion

IgG comes in many different subclasses – most of which have inherently different functions from one another, but are all an integral part of the body’s immune system.

Most of the time, the actions of these immunoglobulins are useful, but they can sometimes become over-active and cause autoimmune diseases, which are typically difficult to treat, and usually have no direct cure.

Learning more about the different IgG subclasses could be a means to discovering new clinical treatments and cures for a range of autoimmune disorders, as well as developing methods to make the host immune system more efficient in a given organism – thus reducing the need for current clinical treatments, such as antibiotics.

Further Reading

• All Antibody Content
• Antibody – What is an Antibody?
• Antibody Forms
• Antibodies in Medicine
• Antibody Structure

Written by

Phoebe Hinton-Sheley

Phoebe Hinton-Sheley has a B.Sc. (Class I Hons) in Microbiology from the University of Wolverhampton. Due to her background and interests, Phoebe mostly writes for the Life Sciences side of News-Medical, focussing on Microbiology and related techniques and diseases. However, she also enjoys writing about topics along the lines of Genetics, Molecular Biology, and Biochemistry.