1. In this prospective cohort, the included kidney transplant recipients will receive regular post-transplantation care, including routine therapeutic drug monitoring of tacrolimus and screening for BK viremia. No extra medications beyond the standard of care will be given to the patients in this study.
2. Whole blood will be drawn from kidney transplant recipients on day 30, day 180, and at the time of BK viremia. PBMCs will be separated and incubated with BK viral peptides.
3. PBMCs will be stained for surface marker of activated T cells and intracellular cytokines 4. Phenotypic features of T cells will be analyzed under flow cytometry and correlated with the occurrence of BKV infection and immunosuppressive medications.

Blood will be drawn and investigated for BKV-specific T cell response.

- To examine the capability of flow cytometry in predicting post-kidney transplant BKV infection, by the analyses of intracellular cytokines (interleukine-2) and surface activation markers (CD25) of T cells

To examine the chonological change of BKV-specific immune response after transplantation, by the analyses of intracellular cytokines (interleukine-2) and surface activation markers (CD25) of T cells.