IGG, GT X HU, HRP-2ML IgG is a monomeric immunoglobulin, consists of two heavy chains (gamma) and two light chains. Each molecule has two antigen binding sites. IgG is the most abundant immunoglobulin. It is approximately equally distributed in blood and in tissue liquids, constituting 75% of serum immunoglobulins in humans. This is the only isotype that can pass through the human placenta, providing protection to the fetus in its first weeks of life before its own immune system has developed. There are 4 subclasses: IgG1 (66%), IgG2 (23%), IgG3 (7%) and IgG4 (4%).