What are the functions of IgG (Immunoglobulin G) immunoglobulins?

Functions of IgG Immunoglobulins

IgG immunoglobulins are the most abundant antibody isotype that protect against infections through multiple mechanisms including neutralization of pathogens, opsonization, complement activation, and immunomodulatory functions.

Primary Functions of IgG

• Pathogen Neutralization: IgG antibodies bind to and neutralize foreign objects such as bacteria and viruses, preventing them from causing infection 1
• Response to Specific Antigens: Different IgG subclasses respond to different types of antigens - IgG1 and IgG3 primarily mediate responses to protein antigens, while IgG2 mediates responses to polysaccharide antigens 2
• Complement Activation: IgG1 and IgG3 effectively activate the classical complement pathway, while IgG4 can activate complement through the alternative pathway 2
• Opsonization: IgG antibodies coat pathogens (opsonization), marking them for phagocytosis by immune cells 3
• Long Half-Life: IgG antibodies have a long half-life in circulation, providing extended protection against pathogens 3
• Tissue Penetration: IgG can penetrate virtually all tissues in the body, allowing for widespread protection 3

IgG Subclasses and Their Functions

IgG consists of four subclasses (IgG1, IgG2, IgG3, and IgG4), each with distinct functions:

• IgG1:

  • Most abundant subclass (approximately 60% of total IgG) 4
  • Primary response to protein antigens 2
  • Strong complement activation 2

• IgG2:

  • Mediates response primarily to polysaccharide antigens, particularly encapsulated bacteria 2
  • Important for protection against pneumococci and other encapsulated bacteria 4

• IgG3:

  • Often works in conjunction with IgG1 4
  • Strong complement activation and binding to immune cells 2

• IgG4:

  • Present in very low concentrations, especially in children under 10 years 4
  • Role less clear but may have both protective and sensitizing functions 2

Clinical Significance of IgG

• Protection Against Infections: IgG deficiency leads to increased susceptibility to recurrent bacterial infections, particularly from encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis 4, 2

• Immunomodulation: IgG antibodies play a role in maintaining self-tolerance and resolving inflammation, with deficiencies potentially leading to autoimmune conditions 3

• Therapeutic Applications:

  • IgG replacement therapy is used in primary and secondary immunodeficiency disorders 5
  • Intravenous immunoglobulin (IVIG) is used as immunomodulatory therapy for autoimmune and inflammatory disorders 6
  • Specific high-titered IgG preparations are used for protection against certain viruses or bacterial toxins 5

• Maternal-Fetal Protection: IgG is the only antibody class that crosses the placenta, providing passive immunity to the fetus 4

Clinical Implications of IgG Deficiency

• Recurrent Infections: Patients with IgG deficiencies experience recurrent bacterial infections, particularly of the respiratory and digestive tracts 2

• Impaired Vaccine Response: Deficiencies in specific IgG subclasses can result in poor responses to vaccines, especially polysaccharide vaccines in the case of IgG2 deficiency 4

• Association with Other Conditions: IgG subclass deficiencies are often associated with IgA deficiency, allergic diseases, autoimmune conditions, and certain central nervous system disorders 2

• Treatment Considerations: IgG replacement therapy may benefit patients with clinically significant deficiencies and recurrent infections 2, 1

IgG in Therapeutic Applications

• Replacement Therapy: Administered as intravenous (IGIV) or subcutaneous (SCIG) infusions for patients with primary or secondary immunodeficiencies 5

• Immunomodulation: High-dose IgG therapy is used for its anti-inflammatory and immunomodulatory effects in autoimmune conditions 6

• Specific Prophylaxis: Used for prevention of certain infections in high-risk populations 4

• Post-Exposure Prophylaxis: Specific IgG preparations can be used after exposure to certain pathogens 4