IgG – Immunoglobulin G Explained
IgG (Immunoglobulin G) is the most abundant antibody class in human blood and a key component of the immune system, providing long-term protection against infections.
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IgG (Immunoglobulin G) is the most abundant antibody class in human blood and a key component of the immune system, providing long-term protection against infections.
What is IgG?
IgG (Immunoglobulin G) is the most abundant antibody class found in human blood and other body fluids, accounting for approximately 70–80% of all circulating antibodies. Antibodies are proteins produced by the immune system to identify and neutralize foreign substances such as bacteria, viruses, and other pathogens. IgG plays a central role in long-term immunity and immunological memory.
Structure of IgG
IgG has a characteristic Y-shaped molecular structure composed of two heavy chains and two light chains linked by disulfide bonds. Each molecule has two antigen-binding sites that can specifically recognize and attach to foreign molecules known as antigens. There are four subclasses: IgG1, IgG2, IgG3, and IgG4, each with slightly different functional properties and abundances in the bloodstream.
Functions of IgG
- Neutralization of pathogens: IgG antibodies bind directly to viruses, bacteria, and their toxins, preventing them from infecting host cells.
- Opsonization: IgG coats pathogens, marking them for destruction by phagocytic cells such as macrophages and neutrophils.
- Complement activation: IgG can trigger the complement system, a cascade of proteins that directly destroys pathogens and enhances the overall immune response.
- Immunological memory: Following infection or vaccination, specific IgG antibodies are produced and persist, enabling a rapid and effective immune response upon re-exposure to the same pathogen.
- Placental transfer: IgG is the only antibody class capable of crossing the placental barrier, providing passive immunity to the newborn (also known as passive or maternal immunity).
IgG in Diagnostics
Measuring IgG levels in the blood is an important diagnostic tool. Elevated IgG levels may indicate a chronic infection, autoimmune disease, or lymphoma. Low IgG levels may suggest an immune deficiency such as selective IgG deficiency or hypogammaglobulinemia, which can increase susceptibility to recurrent infections.
When is an IgG Test Performed?
- To investigate recurrent or severe infections for underlying immune deficiency
- To assess the immune response after vaccination (e.g., hepatitis B or COVID-19)
- When an autoimmune disease is suspected
- For monitoring of known immune disorders
- In the diagnosis and monitoring of plasma cell disorders such as multiple myeloma
IgG Subclasses
The four IgG subclasses have distinct biological roles:
- IgG1: The most prevalent subclass, primarily responding to protein antigens such as viral proteins.
- IgG2: Important for defense against encapsulated bacteria (e.g., Streptococcus pneumoniae) with polysaccharide capsules.
- IgG3: Highly effective at activating the complement system; responds rapidly during acute infections.
- IgG4: Involved in allergic responses and chronic inflammatory conditions; also observed following allergen immunotherapy.
IgG in Autoimmune Diseases
In various autoimmune diseases, the immune system mistakenly produces IgG antibodies directed against the body's own tissues. Examples include autoantibodies such as anti-double-stranded DNA (anti-dsDNA) antibodies in systemic lupus erythematosus or rheumatoid factor in rheumatoid arthritis. Testing for specific IgG autoantibodies is therefore a key element in diagnosing autoimmune conditions.
IgG in Therapy
Intravenous or subcutaneous immunoglobulin preparations (IVIG/SCIG), which consist predominantly of IgG, are used therapeutically in the management of:
- Primary and secondary immune deficiencies (replacement therapy)
- Certain autoimmune diseases (e.g., Guillain-Barre syndrome, immune thrombocytopenia)
- Inflammatory neuropathies such as chronic inflammatory demyelinating polyneuropathy (CIDP)
References
- Janeway CA, Travers P, Walport M et al. - Immunobiology: The Immune System in Health and Disease. 9th edition. Garland Science, New York, 2016.
- World Health Organization (WHO) - Primary immunodeficiency diseases: Report of a WHO Scientific Group. WHO Technical Report Series, 2017.
- Schwartz RS - Therapeutic use of intravenous immunoglobulin (IVIG). New England Journal of Medicine, 2001; 345(19):1401-1409.
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