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CD21 is involved in the complement system. It binds to iC3b （inactive derivative of C3b）, C3dg, or C3d.B cells express CD21 receptors on their surfaces, allowing the complement system to play a role in B-cell activation and maturation. CD21 interacts with CD19 and on mature B cells forms a complex with CD81 （TAPA-1）. The CD21-CD19-CD81 complex is often called the B cell co-receptor complex, because CD21 binds to opsonized antigens through attached C3d （or iC3b or C3dg） when the B-cell receptor binds antigen. This results in the B cell having greatly enhanced response to the antigen. Epstein-Barr virus （EBV） can bind CD21, enabling EBV to enter and infect B cells.