Nonimmunological host defenses: a review

Nonimmunological defenses are very diverse in type. Some are directed against already transformed cells and belong to mechanisms of containment. Others exert a surveillance by preventing or inhibiting initial events of carcinogenesis. Chalones and oncolytic factors in sera and exudates are agents of containment. Under appropriate circumstances, the autoxidation of thiols and the formation of mixed disulfides lead to destruction of tumor cells in vitro and in vivo. Both processes involve the generation of superoxide radicals and of hydrogen peroxide which, in turn, activate the peroxide:peroxidase:halide system. Thiol:disulfide ratios and interchange codetermine the antioxidative activity of cellular membranes, thus bearing on carcinogenesis. Many aliphatic and aromatic antioxidants are endowed with anticarcinogenic properties. The fact that they are inhibitors of free radical processes corroborates the increasingly evident role of free radicals in carcinogenesis. Endogenous antioxidants and exogenous ones in foods are agents of surveillance. Antioxidant activity, linked with the ergastoplasm, points to a homeostatic mechanism that prevents self-accelerating chain reactions from leading to membrane damage or to carcinogenesis. Carcinogens can also be inactiviated by microsomal enzymes belonging to an overall mechanism of detoxification. Activity levels of these systems depend on diet and state of nutrition. They may be naturally very low, but they can be increased with various inducers.