A key problem with kidney transplantation is the potential for the patient’s immune system to reject the kidney. Hence all transplant patients are given medications to suppress their immune systems. If their immune system is too suppressed, patients are at risk of severe infection; if it is not suppressed enough, they are at risk of rejecting the kidney. Our current methods for monitoring the degree of immune suppression are crude and episodes of both severe infection and organ rejection are common. There is currently no simple and practical blood test which quantifies how suppressed the immune system is. Such a blood test would allow us to get the doses of immunosuppressive drugs just right in each patient, using an individually tailored approach.