{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"CD21, also known as CR2, complement component (3d/Epstein-Barr virus) receptor 2, is an integral membrane glycoprotein of molecular weight 140 kDa, involved in the complement system. CD21 binds to C3d. B-cells have CR2 receptors on their surfaces, allowing the complement system to play a role in B-cell activation and maturation. Epstein-Barr virus also binds to CD21 during infection to gain entry into B-cells."}]}]}