Explain the structure and functions of IgG immunoglobulin. (IAS 2019/15 Marks)

Introduction

Immunoglobulin G (IgG) is a type of antibody that plays a crucial role in the immune system's defense against pathogens. It is the most abundant antibody in the human body, accounting for approximately 75% of all antibodies. IgG is produced by plasma cells and is involved in both the innate and adaptive immune responses.

Structure of IgG:

• Basic Composition: IgG is a Y-shaped, bivalent glycoprotein composed of four polypeptide chains:
  
  • Two identical heavy (H) chains
  • Two identical light (L) chains
  • These chains are held together by disulfide bonds.
• Heavy Chains: IgG is characterized by the presence of gamma (γ) heavy chains, which distinguish it from other antibody classes.
• Light Chains: There are two types of light chains: kappa (κ) and lambda (λ). Both types are structurally similar but differ in their amino acid sequences.
• Fab Region: The two arms of the Y-shape consist of the Fab (Fragment antigen-binding) region, which binds specifically to the antigen. This region is responsible for antigen recognition and neutralization.
• Fc Region: The stem of the Y contains the Fc (Fragment crystallizable) region, which interacts with cell receptors and complement proteins, enabling immune responses.

Functions of IgG:

• Antigen Neutralization: IgG antibodies can bind to toxins, viruses, and other pathogens, neutralizing them and preventing them from causing harm to the body.
• Opsonization: IgG enhances phagocytosis by coating pathogens, marking them for destruction by phagocytes (e.g., macrophages, neutrophils).
• Complement Activation: The Fc region of IgG can activate the classical pathway of the complement system, which leads to the lysis of pathogens and enhanced immune response.
• Immunological Memory: IgG is critical for long-term immunity. Upon first exposure to an antigen, IgG levels rise slowly. On subsequent exposures, the immune system produces IgG rapidly due to memory B cells, providing faster and stronger protection.
• Placental Transfer: IgG is the only immunoglobulin that can cross the placenta, providing passive immunity to the fetus during pregnancy. This passive immunity protects the newborn from infections during the first few months of life.
• Antibody-Dependent Cellular Cytotoxicity (ADCC): IgG can bind to infected cells or tumor cells, signaling immune cells like Natural Killer (NK) cells to destroy these targets.
• Neutralization of Microorganisms: IgG antibodies are particularly effective in neutralizing a wide variety of microorganisms, including bacteria and viruses, by preventing their entry into host cells.

Conclusion

IgG is a versatile antibody that plays a crucial role in the immune response against pathogens. Its structure allows for specific antigen binding, while its functions include neutralization, opsonization, complement activation, ADCC, and maternal immunity.