The classical immunoglobulin class also known as 7S IgG (150 kD). Composed of two identical light and two identical heavy chains, the constant region sequence of the heavy chains being of the g type. The molecule can be described in another way as being composed of two Fab and an Fcfragment. The Fabs include the antigen combining sites; the Fc region consists of the remaining constant sequence domains of the heavy chains and contains cell-binding and complement-binding sites. IgGs act on pathogens by agglutinating them, by opsonizing them, by activating complement-mediated reactions against cellular pathogens and by neutralising toxins. They can pass across the placenta to the foetus as maternal antibodies, unlike other Ig classes. In humans four main subclasses are known, IgG2 differs from the rest in not being transferred across the placenta and IgG4 does not fix complement. IgG is present at 8-16 mg/ml in serum.1. A23187
Dictionary of molecular biology. 2004.