Which virulence factor of Staphylococcus aureus helps immune evasion by binding the Fc portion of IgG?

• A. Protein A binds the Fc portion of IgG to inhibit opsonization.
• B. Coagulase forms clots to shield bacteria.
• C. Catalase neutralizes reactive oxygen species.
• D. Hyaluronidase degrades tissue.

Answer: (A)

Binding the Fc portion of IgG is a trick that helps Staphylococcus aureus slip past immune defenses. On the bacterial surface, a protein binds the Fc region of antibodies rather than the antigen-binding Fab region. This reverses the antibody’s orientation, so Fc receptors on neutrophils and macrophages can’t engage effectively. Without proper Fc engagement, opsonization is reduced and phagocytosis is less efficient, allowing the bacteria to evade immune clearance. This mechanism also interferes with how antibodies help activate complement, further hindering immune attack.

Other factors like coagulase promote clot formation to shield bacteria, catalase helps neutralize hydrogen peroxide from phagocytes, and hyaluronidase breaks down tissue to assist spread. They aid infection in different ways, but they don’t block Fc-mediated antibody recognition the way Protein A does.