A young patient with severe recurrent pyogenic bacterial infections, but with normal T-cell and B-cell numbers, arrives at the hospital. Testing reveals that this patient's CD4 T-helper cells have a defect in CD40 ligand. As a result, humoral immunity evaluation reveals a significant elevation in the levels of which immunoglobulin that is present as a monomer on B-cell surfaces, as a pentamer in serum, and is initially seen in the primary immune response?
IgG IgA IgM IgD