Description: IgG is the major immunoglobulin in blood, lymph fluid, cerebrospinal fluid, and peritoneal fluid and a key player in the humoral immune response. The Fc portion of IgG, but not F(ab')2 or Fab fragments, can cross the placenta of a mother to enter the fetal circulation providing the fetus with postpartum protection.
IgG molecules are able to react with Fc-gamma receptors that are present on the surfaces of macrophages, neutrophils, natural killer cells, and can activate the complement system.
The binding of the Fc portion of IgG to the receptor present on a phagocyte is a critical step in the opsonizing property IgG provides to the immune response. Phagocytosis of particles coated with IgG antibodies is a vital mechanism to cope with microorganisms.
IgG is produced in a delayed response to an infection and can be retained in the body for a long time. The longevity in serum makes IgG most useful for passive immunization by transfer of this antibody. Detection of IgG usually indicates a prior infection or vaccination.