Protein A (SPA) is an immunoglobulin (Ig)-binding protein found on the bacterial surface and can be freely secreted into the extracellular environment. It binds to the Fc region of antibodies and the Fab region of B cell receptors, thereby blocking opsonophagocytosis and inducing B cell death in vitro. Protein A is capable of inhibiting opsonization and phagocytosis in vitro.