The receptor for the virus on epithelial cells and B lymphocytes is the CD21 molecule, formerly called CR2, which is also the receptor for the C3d component of complement.9In vitro the infection of B lymphocytes by EBV determines a continuous cell proliferation, which results in a cellular immortalisation; the infection of epithelial cells, instead, results in an active replication, with production of virions and host cell lysis.10In vivo there are other factors in addition to CD21 that are important to determine the infection. This evidence concerns the gene CR2 and infection.