The major immunoglobulin class, present at 8–16 mg/mL in serum, also known as 7S IgG (150 kDa). Each IgG molecule comprises two identical light chains and two identical heavy chains, usually schematically shown as Y-shaped with the two arms, composed of the variable regions of both the light and heavy chains, having the antigen-binding sites and the tail, comprising the constant regions of the γ-type heavy chains, forming the Fc region that has cell-binding and complement-binding sites. By proteolytic cleavage the molecule can be split into two Fab fragments and an Fc fragment. IgGs are functionally bivalent and will neutralize toxins, agglutinate, or opsonize pathogens; binding of immunoglobulin can trigger complement activation. IgG can be transferred from mother to fetus, conferring immune resistance in the immediate neonatal period. Four main subclasses are known; IgG2 is not transferred across the placenta and IgG4 does not fix complement.
Subjects: Medicine and Health.