Basic components of the immune system part 16

In contrast, IgG is a smaller molecule that penetrate easily into tissue. There are four majors classes of IgG: IgG1 and IgG3  activate complement efficiently and clear most protein antigens, including the removal of microorganisms by phagocytic cells. In contrast, IgG2 and IgG4 react mostly with carbohydrate antigens and are relatively poor opsonins. This is the only molecule that crosses the placenta to provide immune protection to the neonate.