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( 1, 2, 3, 4, 5 ) pentameric ( IgM, IgD, IgG, IgE, IgA ) or ( 1, 2, 3, 4, 5 ) ( IgA, IgM, IgG, IgD, IgE ) ( in close proximity to each other, on opposite sides of the membrane, anywhere on the membrane ) are required to activate the classical pathway.