Specificity: Recognizes human CD21, a 145kD cell surface glycoprotein expressed by mature B lymphocytes, follicular dendritic cells, subsets of normal thymocytes and subsets of T cells. The CD21 molecule acts as a receptor for complement components C3d, C3dg and iC3b, as well as for Epstein Barr Virus. CD21 is part of the signalling complex that is linked to CD19, CD81 and phosphatidylinositol-3-kinase.