A 62-year-old woman presents with fever, cough, sputum production, and pleuritic chest pain. CXR reveals a right middle lobe infiltrate, and she is started on antibiotics for the treatment of pneumonia. Her sputum Gram stain is positive for S pneumoniae. Which of the following immunologic mechanisms is the most specific host defense against pneumococcal infection?
intact splenic function intact complement function IgG antibody directed against capsular antigens alveolar macrophages