It is the smallest antibody. It is present in the blood, lymph and intestines. 75%-80% of the human body is IgG. IgG is the only antibody that can penetrate the placenta and enter the fetus. It inhibits bacteria and viruses in the process of opsonization. It also works in detoxification. It is of four types. Namely- IgG1, IgG2, IgG3 and IgG4.