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?

The most specific immunologic defense is directed at capsular antigens and is serotype specific. Antibodies are not naturally occurring, but are the result of prior colonization, infection, or vaccination. Intact spleen, complement and macrophage function is important in clearance of pneumococci from sterile areas, but specific immunoglobulin G (IgG) antibody coating of a pneumococcal polysaccharide capsule is essential for macrophages to ingest and kill the bacteria.