複選題6. Which of the following statements regarding "Immunoglobulins" are CORRECT? (A) IgM is a pentamer, which is involved in early primary immune response (B) Class switch recombination generates antibodies with different light-chain constant regions (C) Anti-PD-1 antibody has been developed with potential to restore the latent anti-tumor immunity for several types of tumors (D) Monoclonal antibodies are produced by fusing mouse spleen cells with primary B cells (E) IgG can cross placenta and provide passive immunity to new born