The study drug is an antibody that targets a molecule called PD-1. PD-1 is used to turn down the immune system. In general, this is used by the body to prevent the immune system from being too active. However, several cancers appear to use this pathway to prevent the immune system from attacking them. The theory behind this study is that by blocking PD-1, we may be able to prevent the cancer from hiding from the immune system and allow the immune system to attack the cancer more effectively. There is evidence that the type of lymphoma the participant have may use PD-1 to escape the immune system.