Method: The anti-hepatitis B surface antigen (HBs) titer will be checked in patients who received liver transplantation > 1 year ago. If the titer is < 10 IU/L, a booster dose will be administered. The humoral (anti-HBs) and cellular immunity (by ELISPOT to assay T and B cell specific proliferation) and cytokine assay will be done in these patients before and after the booster dose. A three-year follow-up will be performed to monitor the HBV infection in these patients.