Potentially damaging species (reactive oxygen, nitrogen and chlorine species) arise as by-products of metabolism and as physiological mediators and signalling molecules. Levels of these species are controlled by the antioxidant defence system. Several components of this system are micronutrients (e.g. vitamins C and E) or are dependent upon dietary micronutrients (e.g. CuZn and Mn superoxide dismutase). The antioxidant defences act as a coordinated system where deficiencies in one component may affect the efficiency of the others. Oxidative stress may be an important factor in infection if micronutrients are deficient.