BK virus is endemic in the UK adult population. Although it causes little or no disease in healthy people the virus is known to persist in the kidney in particular. Reactivation of the virus is linked with immune suppressive conditions and this reactivation occurs in around 20% of kidney transplant patients and can lead to significant damage to the transplanted organ. While the level of the virus itself can be measured there is currently no test available that indicates whether or not the patient is able to fight off the infection successfully through their T cell response. This project is therefore aimed at proving the principle for the design of such a test so that kidney transplant recipients can be managed for the infection more effectively, leading to less transplanted organ loss due to this infection.