A 35-year-old woman with a history of traumatic splenectomy presents to the emergency department with sudden-onset fever, headache, and neck stiffness. She also has petechial lesions on her trunk and extremities. Lumbar puncture reveals neutrophilic pleocytosis and Gram-negative intracellular diplococci. Blood cultures grow Neisseria meningitidis. Which bacterial structure is primarily responsible for the organism’s ability to resist phagocytosis in this patient?