There is no established treatment for BK virus infection. Nevertheless, in kidney transplanted patients diagnosed with BK virus infection, immunosuppression is reduced to allow the patients own immune system to handle the virus. However, reduction of immunosuppression has not been associated with rejection. This indicate that these patients were over-immunosuppressed, predisposing them to BKVN. Therefore, to compare the degree of immunosuppression in BKVN patients (over-immunosuppressed) to other patients (not over-immunosuppressed) could yield interesting information. One possibility would be to quantify these patients specific cellular immune response to BK virus but also to other viruses (T cell reactivity).