A 58-year-old man who underwent splenectomy following a motor vehicle accident 6 months ago presents to the emergency department with fever, productive cough, and pleuritic chest pain. Chest radiograph reveals a right lower lobe consolidation. Blood cultures grow alpha-hemolytic, optochin-sensitive gram-positive diplococci. The patient’s peripheral smear shows Howell-Jolly bodies. Which of the following bacterial structures is most responsible for this organism’s ability to resist phagocytosis and cause invasive disease in asplenic patients?