The HB5 monoclonal antibody reacts with human CD21, an approximately 145 kDa
type I transmembrane protein. CD21 is expressed by mature B cells, folliculate
dendritic cells and a subset of epithelial cells and is a receptor for complement
component C3d and Epstein-Barr virus (EBV). CD21, in association with CD19 and CD81,
participates in the multimolecular complex with BCR and is involved in B cell activation.