A protein that resides in the microbial wall of staphylococcus aureus and interferes with opsonization by binding to the Fc portion of immunoglobulin. The protein has a deleterious effect on the epithelial cells that line the respiratory tract, and plays a role in the induction of pneumonia. Protein A also initiates polymorphonuclear cell migration into airway passages via TNFR1 activation.