Involved in countering the first line of host defense mechanisms. Efficiently inhibits opsonization, phagocytosis and killing of S.aureus by human neutrophils. Acts by binding and stabilizing human C3 convertases (C4b2a and C3bBb), leading to their inactivation. The convertases are no longer able to cleave complement C3, therefore preventing further C3b deposition on the bacterial surface and phagocytosis of the bacterium. Also prevents C5a-induced neutrophil responses. Expressed maximally during the exponential growth phase. Up-regulated by agr and sae loci. Down-regulated by sigma B factor. Belongs to the SCIN family.