The present invention provides methods of treating cancer in a subject, preventing or delaying relapse to a cancer in a subject in remission, prolonging remission from cancer, increasing survival, and decreasing or alleviating cancer symptoms comprising a) administering histamine dihydrochloride and an inhibitor of the Programmed cell Death protein 1 (PD-1)/Programmed Death Ligand 1 (PD-L1) or b) administering an agent that decreases reactive oxygen species (ROS) optionally, together with a histamine receptor agonist. The present invention also provides methods of predicting the efficacy of a cancer treatment based on a re-distribution of cytotoxic T cells, frequency of NK cells, or other biochemical changes, and related methods of preventing relapse to cancer and for prolonging remission from a cancer. Related kits and compositions are also provided.
