Evidence for increased levels of T cell exhaustion in effector memory CD 4+ T cells of HL patients. PBMC were stained with specific fluorescent conjugated antibodies against T cell markers (CD3, CD4) together with differentiation markers (CD45RO, CD27) and PD1 and analyzed using FACS (LSR-II). The proportion of PD1+ T cells was determined in CD4+RO+CD27- T cells