A 34-year-old woman underwent a splenectomy following a motor vehicle accident. Six months later, she presents with fever, pleuritic chest pain, and a productive cough. Chest radiography reveals a right lower lobe consolidation, and blood cultures grow alpha-hemolytic, optochin-sensitive, gram-positive diplococci that form mucoid colonies. In vitro assays show her neutrophils poorly phagocytose these bacteria unless the organisms are preincubated with specific antiserum. Which structural component of this organism is primarily responsible for its resistance to phagocytosis in the absence of specific antibody?