Understanding Immunoglobulin G (IgG)

What is Immunoglobulin G?

Immunoglobulin G (IgG) is the most abundant type of antibody found in the blood and extracellular fluid, playing a crucial role in the immune response. It constitutes about 75-80% of serum antibodies in humans and is vital for identifying and neutralizing pathogens such as bacteria and viruses.

Structure of IgG

IgG is characterized by its structure known as a Y-shaped molecule. It consists of:

• Two heavy chains of approximately 50 kDa each
• Two light chains of approximately 25 kDa each
• A hinge region allowing flexibility in binding antigens

This structure enables IgG to bind effectively to antigens and immune cells, thus activating various immune processes.

Functions of IgG

IgG performs several critical functions, including:

• Neutralization: IgG can neutralize toxins and viruses, preventing them from entering or damaging cells.
• Opsonization: It tags pathogens for destruction by phagocytes, enhancing their elimination from the body.
• Complement Activation: IgG initiates the complement cascade, leading to the lysis of target cells.
• ADCC (Antibody-Dependent Cell-mediated Cytotoxicity): IgG can engage natural killer cells to kill infected cells.

Types of IgG

There are four subclasses of IgG, known as IgG1, IgG2, IgG3, and IgG4, each with differing abilities to bind to various pathogens:

• IgG1: The most prevalent subclass, particularly efficient in neutralizing toxins and viruses.
• IgG2: More effective against polysaccharide antigens present on some bacteria.
• IgG3: Known for its strong ability to activate the complement system.
• IgG4: Less capable of activating complement but can modulate immune responses.

Clinical Significance of IgG

Measurements of IgG levels are often used in clinical diagnostics to:

• Assess immune status in infections.
• Diagnose autoimmune diseases.
• Monitor responses to vaccinations.
• Identify immunodeficiency disorders.

Abnormal IgG levels can indicate various health conditions, making it an essential component in clinical immunology.