CD4 and post-transplant lymphoproliferative disease: The identification of low endogenous CD4+ T-cell numbers as important risk factor for the development of EBV-associated diseases in immunosuppressed patients [38], and of better clinical responses in patients with PTLD receiving EBV-specific T-cell lines that contained higher proportions of CD4+ T cells [10], have implied an important role for CD4+ T cells in the control of EBV-driven lymphoproliferation.