A 32-year-old woman presents to the immunology clinic with a 6-month history of recurrent sinusitis and bronchitis. Serum protein electrophoresis shows a monoclonal spike. Flow cytometry reveals CD19+ B cells producing only kappa light chains. Serum IgM levels are markedly elevated (850 mg/dL; normal <200), while IgG and IgA are normal. A bone marrow biopsy shows 40% plasma cells. Which antibody class is predominantly elevated, and what is the structural basis for its poor opsonization compared to IgG?