Question
IgG (Immunoglobulin G)
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 two identical heavy chains (γ) and two identical light chains, forming a “Y” shape with two antigen-binding (Fab) arms and one Fc region; four subclasses (IgG1–IgG4).
Key properties/functions: only immunoglobulin class that crosses the placenta, providing passive protection to the newborn; predominant antibody of the secondary immune response, persisting long-term; efficiently opsonizes pathogens; activates the classical complement pathway; mediates antibody-dependent cellular cytotoxicity (ADCC); neutralizes toxins and viruses.
Clinical significance: IgG serology is used to assess immune status; IgG antibody titres generally indicate past rather than current/acute infection, an important distinction in interpreting tests like the TORCH panel.