Since it was recognized that the Epstein Barr Virus (EBV) could be used in vitro to infect human B lymphocytes and produce immortalized polyclonal B cell lines in culture, this technique has been used with great effect to facilitate immunological and molecular studies. EBV belongs to the herpesvirus family and is the main agent that causes human mononucleosis. The EBV infects B lymphocytes and immortalizes and transforms the cells into long-term growing EBV-bearing lymphoblastoid cell lines (LCLs), which consistently express the viruses.