Microbiology for COMLEX Level 1 Practice Test

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Which virulence factor of Staphylococcus aureus binds the Fc region of IgG, inhibiting opsonization?

Protein A

Staphylococcus aureus uses a protein that binds the Fc portion of IgG to blunt immune recognition. Protein A attaches to the Fc region of IgG, leaving the Fab portion free to bind antigen while the Fc region is hidden from Fc receptors on phagocytes. This misorients antibodies and prevents opsonization and effective phagocytosis, helping the bacteria avoid immune clearance.

Other listed factors have different roles: coagulase traps bacteria in fibrin clots, hemolysins lyse cells (not specifically affecting antibody orientation), and catalase detoxifies hydrogen peroxide produced by neutrophils. None of these blocks opsonization by binding the Fc region.

Coagulase

Hemolysin

Catalase