Molecular sites on or in b-lymphocytes, follicular dendritic cells, lymphoid cells, and epithelial cells that recognize and combine with complement c3d. Human complement receptor 2 (CR2) serves as a receptor for both C3dg and the gp350/220 glycoprotein of herpesvirus 4, human, and binds the monoclonal antibody OKB7, which blocks binding of both ligands to the receptor.