Staphylococcus aureus expresses two proteins on its surface: staphylococcal protein A (SPA) and staphylococcal immunoglobulin-binding protein A (Sbi) (Zhang et al. 1998 ) that can bind to the Fc portion of IgG and thus prevent complement activation and Fc receptor-mediated phagocytosis. Sbi can also block binding of C1q and thus prevent activation. Similarly, group C and G streptococcal bacteria express protein G that can bind the Fc portion of IgG. Furthermore, Staphylococcus aureus expresses receptors that bind plasminogen, and Staphylokinase secreted by Staphylococcus aureus cleaves bound plasminogen to plasmin. Bacterial surface bound plasmin can degrade IgG and the opsonin C3b, and thus evade the complement system. Further, Staphylococcus aureus-secreted staphylococcal complement inhibitors (SCINs) inhibit complement activation. These SCINs bind to C3 convertases and prevent subsequent steps involved in complement activation, and thus prevent opsonization, phagocytosis and cell lysis.