Immunoglobulin G (IgG) is a tetrameric immunoglobulin, built of two heavy chains γ an two light chains. Each IgG has two antigen binding sites. It is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids, constituting 75% of serum immunoglobulins in humans. IgG molecules are synthesised and secreted by plasma B cells.