Free radicals: biology and relevance to disease.

Free radicals are the major mediators of the bactericidal and cytotoxic actions of polymorphonuclear leucocytes, macrophages and monocytes, and can stimulate lymphocytes mitogenically. Hydrogen peroxide induces interferon production by human macrophages and activates NK (natural killer) cells; interferon can in turn enhance free radical generation in the cells. Anti-cancer drugs such as doxorubicin, vincristine and adriamycin; radiation; and haematoporphyrin derivative-induced photosensitization all can augment free radical generation and thus, cause tumour cell lysis. Our recent studies suggest that some polyunsaturated fatty acids such as gamma-linolenic acid, arachidonic acid and eicosapentaenoic acid can selectively kill tumour cells but not normal cells in vitro by virtue of their capacity to augment free radical generation in the tumour cells. Thus, free radicals have both harmful and beneficial actions.