Medicine & USMLE

Immunoglobulin G (IgG)

  1. Immunoglobulin A (IgA)
  2. Immunoglobulin G (IgG)
  3. Immunoglobulin M (IgM)
  4. Immunoglobulin E (IgE)
  5. Immunoglobulin D (IgD)


Immunoglobulin G (IgG) is the most abundant type of antibody in blood important in fighting off pathogens in infection. The production of IgG is delayed after exposure to a new antigen, due to the need for B-cells to undergo class-switching from IgM. Because it circulates as a small monomer, IgG is the main antibody able to cross the placenta from the mother to the baby, which makes it pathogenic in diseases like erythroblastosis fetalis. IgG antibodies help clear pathogens from our bodies in three main ways. First, IgG can opsonize microbes, labeling them for ingestion for phagocytic cells. Second, they can fix complement, activating complement by the classical pathway to kill the pathogen and attract other immune cells to the site. Finally, IgG can directly neutralize microbes or toxins produced to prevent further harm. 

Key Points

  • Immunoglobulin G (IgG)
    • Most abundant antibody in serum
    • Production is delayed/slower relative to Immunoglobulin M (IgM)
      • Due to need for B-cell class switching
    • Crosses placenta 
      • provides passive immunity to infants for up to 6 months
      • causative antibody in hemolytic disease of the newborn (erythroblastosis fetalis)
    • Fixes (binds to) complement 
    • Opsonizes material for phagocytosis 
      • Fc receptors (FcR) on phagocytes recognize the Fc region of lgG
    • Directly neutralizes bacterial toxins and viruses
      • e.g. diphtheria antitoxin