A 2-year-old unvaccinated child is brought to the emergency department with a 12-hour history of high fever, difficulty breathing, drooling, and inspiratory stridor. A lateral neck radiograph demonstrates a “thumbprint” sign consistent with epiglottitis. Blood cultures grow small, gram-negative pleomorphic coccobacilli. Which virulence factor of this pathogen is most directly responsible for its ability to resist opsonization and phagocytosis?