21. After activation, naive B cells differentiate into antibody-secreting plasma cells and secrete different isotypes of immunoglobulin or antibodies. Regarding the functions of different isotypes of antibodies, which statement is NOT CORRECT? (A) IgM antibodies usually form pentamer and are produced first in a humoral immune response (B) IgA is the predominant antibody in mucosal sites, such as the intestinal and respiratory tracts (C) IgG efficiently opsonizes pathogens for engulfment by phagocytes and activates the complement system (D) IgE antibody is present in high levels in the blood or extracellular fluid and has a high affinity to bind and active mast cells