Plays a role in the inhibition of both the innate and adaptive immune responses. Possesses two N-terminal domains that bind the Fc region of IgG and two domains that form a tripartite complex with complement factors C3b and CFH. By recruiting CFH and C3b, the secreted form acts as a potent complement inhibitor of the alternative pathway-mediated lysis. Interacts (via sbi-I and sbi-II domains) with the Fc region of mammalian immunoglobulin G (IgG) proteins. Interacts (via sbi-III and sbi-IV domains) with host complement C3. Interacts (via sbi-III and sbi-IV domains) with host CFH (By similarity). Interacts (via sbi-IV domain) with beta-2-glycoprotein 1/APOH (By similarity). Occurs both extracellularly and associated with the cytoplasmic membrane where only the domains I and II are exposed to the extracellular media. Membrane association occurs via binding to lipoteichoic acid. Sbi-I and sbi-II domains provide protection only when anchored to the cell surface, whereas only the secreted sbi-III and sbi-IV domains are biologically active. Belongs to the immunoglobulin-binding protein Sbi family.