Protein A of Staphylococcus aureus contributes to virulence primarily by which mechanism?

• A Cleaving IgA at the hinge region
• B Blocking C3b deposition by degrading the C3 convertase
• C Cross-linking Fab portions of adjacent IgG molecules to activate complement
• D Binding the Fc portion of IgG, preventing opsonisation and phagocytosis ✓

Explanation

Protein A is a cell wall surface protein that binds the Fc region of human IgG1, IgG2 and IgG4, leaving the Fab ends pointing outward. The bacterium therefore wears antibody upside down, so Fc receptors of neutrophils cannot engage it and classical opsonisation fails. Option A describes IgA protease of S pneumoniae, H influenzae and Neisseria, not protein A.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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