Question
Describe immunoglobulin G (IgG)
Answer
Immunoglobulin G (IgG) is the predominant immunoglobulin class in normal human serum, accounting for ~70–75% of total serum immunoglobulin, and is the principal antibody of the secondary (memory) immune response.
Structure: a monomeric molecule with the basic four-chain structure — two identical heavy chains (γ chains) and two identical light chains, linked by disulphide bonds, forming a “Y” shape with two identical antigen-binding (Fab) arms and one Fc (constant) region. Molecular weight ~150,000 Da.
Subclasses: four subclasses (IgG1, IgG2, IgG3, IgG4), differing in hinge-region structure and functional properties (e.g., relative ability to activate complement and bind Fc receptors), with IgG1 and IgG3 being the most efficient complement activators.
Key properties/functions:
- Only immunoglobulin class that crosses the placenta (via active FcRn-mediated transport), providing passive protection to the newborn during the first few months of life — the basis of maternally-derived neonatal immunity.
- Predominant antibody of the secondary immune response — appears later than IgM in a primary response but persists for a much longer duration, providing long-term/durable immunity and forming the basis of serological memory following infection or vaccination.
- Efficiently opsonizes pathogens (Fc portion binds Fcγ receptors on phagocytes), enhancing phagocytosis.
- Activates the classical complement pathway (via C1q binding, though less efficiently per molecule than pentameric IgM, since more molecules bound close together are needed).
- Mediates antibody-dependent cellular cytotoxicity (ADCC) via NK cells.
- Neutralizes toxins and viruses by blocking their attachment to host cells.
- Distributed roughly equally between intravascular and extravascular compartments (unlike IgM, which is largely confined to the intravascular space), owing to its smaller size.
Clinical significance: IgG levels/serology are used to assess immune status (past infection or vaccination), and IgG antibody titres (rather than IgM) generally indicate past rather than current/acute infection, an important distinction in interpreting serological tests (e.g., TORCH panel, dengue serology).