immunoglobulin G (IgG), one of the five classes of antibodies produced by the body and the most abundant antibody isotype found in the circulation. It is synthesized in response to invasions by bacteria, fungi, and viruses. IgG crosses the placenta and can protect the fetus from invading microorganisms. The normal concentration of IgG in serum is 800 to 1600 mg/dL. Compare immunoglobulin A, immunoglobulin D, immunoglobulin E, immunoglobulin M.
