Similarly, complement-inhibiting proteins expressed by viruses such as vaccinia, cowpox and smallpox block C3 convertase formation (for a review see (Tortorella et al. 2000) . These inhibitory proteins contain short consensus repeats (SCR) that are also found in complement proteins (such as Factor B) (Ricklin and Lambris 2007 (CRIT) protein that inhibits C3 convertase by binding to C2 (Cestari Idos et al. 2009; Inal and Sim 2000) . Pathogens can also prevent recruitment of phagocytic cells by inhibiting C5a and C5aR interaction. For example, the Staphylococcus aureus product, chemotaxis-inhibiting protein of Staphylococcus aureus (CHIPS), can bind to C5aR and prevent C5a-mediated signaling, although CHIPS and C5a share hardly any homology but are similar in overall structure and size. Another Staphylococcus aureus product, SSL-7, can bind C5 and inhibit its cleavage preventing MAC formation (Rooijakkers and van Strijp 2007) .