A 65-year-old man with a history of splenectomy presents with fever, productive cough, and dyspnea. On examination, he is febrile and tachypneic with dullness to percussion over the right lower lung field. Chest x-ray reveals right lower lobe consolidation. Blood cultures grow gram-positive, lancet-shaped diplococci. A quellung reaction is positive. Which of the following best explains how the polysaccharide capsule of this organism contributes to its virulence?