An improved composition and method for reducing the formation of oxygen radicals in mononuclear phagocytes is disclosed. Also provided is a method of protecting NK cells from oxidative damage inflicted by radicals produced by mononuclear phagocytes by administering a histamine metabolite, N-alpha-methylhistamine dihydrochloride. Use of N-alpha-methylhistamine dihydrochloride for enhancing the NK cell response to IL-2 is likewise detailed.
