Which of the following immunoglobulin crosses placenta readily and can activate complements?

IgG is the predominant antibody in the secondary response and constitutes an impoant defense against bacteria and viruses. It is the only maternal immunoglobulin that is normally transpoed across the placenta and provides natural passive immunity in the newborn. Only its Fc poion binds to receptors on the surface of placental cells. It is not synthesized by the fetus in any significant amount. It is therefore the most abundant immunoglobulin in newborns. IgG is one of the two immunoglobulins that can activate complement; IgM is the other. IgG is the immunoglobulin that opsonizes. Ref: Levinson W. (2012). Chapter 59. Antibodies. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.