Oxidative stress, resulting from cumulative damage caused by reactive oxygen species (ROS), is present throughout life and is thought to be a major contributor to the ageing process. The immune system is particularly vulnerable to oxidative damage, since many immune cells produce these reactive compounds as part of the body's defence mechanisms. Higher organisms have evolved a variety of antioxidant defence systems either to prevent the generation of ROS or to intercept any that are generated. These defence systems exist in both the aqueous and membrane compartments of cells and can be enzymic or non-enzymic in nature. The enzymes contain metal ions at their active sites - these must be obtained from the diet - while the diet is the source of many non-enzymic components of the body's antioxidant defence system (e.g. antioxidant vitamins).

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