Protein A (SPA) is an immunoglobulin (Ig) binding protein that exists on the surface of bacteria and can be freely secreted into the extracellular environment. Protein A blocks opsonization and phagocytosis and induces B-cell death in vitro by binding to the Fc region of antibodies and the Fab region of B-cell receptors. Protein A has the ability to inhibit opsonization and phagocytosis in vitro^[1].