Understanding IgG: role, disorders & testing
Immunoglobulin G (IgG) is the most common antibody in the human immune system and plays a central role in defending against infections. Altered IgG levels in the blood can indicate various health conditions such as autoimmune diseases, chronic infections, and immune deficiencies. This blog provides a comprehensive overview of the functions of IgG, related disorders, as well as diagnostic and treatment options.
Table of contents
- What is immunoglobulin G (IgG)?
- Difference between IgG and other immunoglobulins
- When and why is an immunoglobulin blood test performed?
- Diseases and immunoglobulin G
- Treatment methods for IgG imbalances
- Summary
What is immunoglobulin G (IgG)?
As an antibody, immunoglobulin G (IgG) is an important component of the human immune system. Antibodies, also known as immunoglobulins, are proteins produced by the immune system to detect and neutralize foreign substances such as bacteria, viruses, and toxins. IgG is the most common type of antibody, accounting for about 75–80% of all immunoglobulins. It is found not only in the blood but also in other body fluids such as lymph. There are several subclasses of IgG (IgG1, IgG2, IgG3, and IgG4), each with specialized functions in the immune system (Britannica, 2024).
Functions of immunoglobulin G (IgG)
Immunoglobulin G (IgG) is not only the most common antibody but also one of the most versatile. Its ability to neutralize various pathogens in different parts of the body makes it indispensable to the immune system. The functions of IgG are listed below.
- Neutralization of pathogens: IgG can neutralize viruses, bacteria, and toxins by binding to them and blocking their ability to infect cells.
- Long-term immunity: IgG is important for immunological memory. This means it helps the body remember previous infections and respond more quickly when the same pathogen appears again. This occurs through specific memory B cells that rapidly produce IgG upon re-exposure.
- Tagging foreign bodies: IgG tags foreign substances so that they can be more easily recognized and eliminated by phagocytes. This process, known as opsonization, serves as a signal for phagocytes to destroy the marked invader.
- Protection for newborns: Immunoglobulin G (IgG) can be transferred from the mother to the unborn child through the placenta, offering protection against infections. This provides important immunity for newborns during the first months of life. After birth, the child begins to produce its own IgG, and the concentration gradually increases.
- Activation of defense proteins: Immunoglobulin G (IgG) activates the complement system, a group of proteins in the blood that help eliminate pathogens and facilitate their recognition by phagocytes.
Source: Angel et al., 2023
Difference between IgG and other immunoglobulins
In addition to immunoglobulin G, there are other antibodies that perform different functions in the body. The following table provides a brief overview of antibody types and their functions:
| Antibody | Function |
|---|---|
| Immunoglobulin G | Most common antibody in the blood and body fluids. Protects the body from viral and bacterial infections by neutralizing pathogens. |
| Immunoglobulin E | Important in fighting parasites and responsible for allergic reactions. Mainly found in the skin, lungs, and mucous membranes. |
| Immunoglobulin D | Found on the surface of B cells. Likely supports B cells in responding to antigens, although this role is not yet fully understood. |
| Immunoglobulin M | First immune response against infections. Also helps coordinate the overall immune reaction. |
| Immunoglobulin A | Present in saliva, tears, mucus, breast milk, and intestinal fluids. Protects against pathogens that enter through food and air. |
Source: Cleveland Clinic, 2022
When and why is an immunoglobulin blood test performed?
An immunoglobulin blood test is performed to measure the concentration of immunoglobulin G (IgG), usually in combination with IgA and IgM, in the blood and to detect abnormalities that may indicate various health problems. The normal values for IgG can vary depending on age, sex, and the testing method used, but in general, reference values for adults range between approximately 6.0–16.0 g/L (grams per liter) (NHS, 2022). This test is done through a simple blood sample and is used to:
- Diagnose infections
- Detect autoimmune diseases
- Identify immune deficiencies
- Evaluate the effectiveness of vaccinations
What do positive IgG antibodies mean?
A positive IgG antibody result means that detectable amounts of IgG are present in the blood. These are specifically directed against a particular pathogen. This indicates that the body has either currently or previously been exposed to this pathogen and has developed an immune response and immunity to it (Medline Plus, 2022).
Diseases and immunoglobulin G
Abnormalities in IgG levels—whether increased or decreased—can indicate various health conditions. The following overview lists some diseases associated with altered IgG levels:
Conditions with elevated IgG levels
- Infectious diseases: During an acute infection, such as the flu, COVID-19, or a bacterial infection, the immune system produces increased amounts of IgG antibodies to fight the pathogens. These antibodies help to eliminate the infection and often provide some level of immunity afterward, meaning specific IgG levels can remain temporarily elevated. Persistently high IgG levels can indicate chronic infections, such as an infection with the Epstein-Barr virus (mononucleosis).
- Autoimmune diseases: In autoimmune diseases, the immune system mistakenly produces IgG antibodies that attack the body’s own cells and tissues. In systemic lupus erythematosus (SLE), these antibodies target various organs and tissues, causing inflammation and damage. Rheumatoid arthritis (RA) is another example in which IgG antibodies play a role, primarily targeting the joints.
- Liver diseases: Certain liver diseases can lead to an increase in IgG levels because the liver is involved in breaking down proteins, including antibodies. In liver cirrhosis, healthy liver tissue is replaced by scar tissue, which impairs liver function and can result in elevated IgG levels, as the damaged liver is less able to break down IgG effectively.
Conditions with decreased IgG levels
A decreased IgG level, or IgG deficiency, is characterized by frequent and severe infections of the respiratory and gastrointestinal tracts, chronic infections, and delayed recovery. The following conditions can lead to low IgG levels:
- Primary immunodeficiencies: These are inherited diseases marked by a malfunction of the immune system, resulting in a lack of IgG antibodies. An example is X-linked agammaglobulinemia, in which the body does not produce B cells and therefore no IgG antibodies, leading to a high risk of recurrent infections.
- Secondary immunodeficiencies: These are acquired conditions that impair the production of IgG and weaken the immune system. HIV/AIDS is a disease that attacks immune cells and reduces the production of IgG and other antibodies, increasing susceptibility to infections. Chemotherapy, a treatment for cancer, can also damage immune cells and affect the production of IgG antibodies.
- Loss of IgG: Certain diseases can lead to the loss of IgG from the body. Nephrotic syndrome is a kidney disorder in which large amounts of protein, including IgG, are excreted in the urine, causing IgG levels to drop. Severe burns can also result in significant loss of proteins and antibodies like IgG through damaged skin.
Sources: NHS, 2022; Cleveland Clinic, 2022
Treatment methods for IgG imbalances
Medical treatment methods
In cases of elevated IgG levels, treatment may include immunosuppressive medications to suppress the excessive immune response. Low IgG levels are often treated by administering immunoglobulins, either intravenously—meaning the antibodies are injected directly into a vein—or subcutaneously—meaning they are injected under the skin. This immunoglobulin therapy increases the antibody levels in the blood and improves immune function. Both approaches aim to balance the immune system and relieve symptoms. However, these therapies are often costly and typically require regular treatment (msdmanuels, 2023).
Lifestyle-based treatment methods
While it is not possible to directly increase IgG levels through lifestyle alone, a healthy lifestyle can strengthen the immune system as a whole. A balanced diet, regular exercise, sufficient sleep, and effective stress management all help support immune function and promote overall health.
Summary
Immunoglobulin G (IgG) is the most common antibody in the human immune system and plays a critical role in defending against infections. Altered IgG levels can indicate various health problems such as autoimmune diseases, chronic infections, and immune deficiencies. Immunoglobulin tests help diagnose these changes and guide appropriate treatment strategies. Treatment may include both medical approaches, such as the administration of immunoglobulins, and lifestyle measures to support immune health.
