Composed of two identical light and two identical heavy chains, the constant region sequence of the heavy chains being of the type. The molecule can be described in another way as being composed of two Fab and an Fc fragment.
Immunoglobulin Gs act on pathogens by agglutinating them, by opsonising 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. Immunoglobulin G is present at 8-16 mg/ml in serum.
Synonyms: gamma globulin.
(30 Mar 1998)
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