The immune system is extremely sensitive to oxidative damage. Many immune cells produce reactive oxygen species as part of the body's defence against infection and depend on adequate protection by antioxidants for optimal function. Many antioxidants can be obtained directly from the diet (e.g. vitamin C, vitamin E, carotenoids and polyphenolic flavonoids) or require micronutrients as integral components of protective enzymes (e.g. selenium, copper and zinc). Whilst a balanced diet should provide sufficient protection, periods of infection, inflammatory conditions or cigarette smoking may tip the balance in favour of oxidative stress. This review summarises some of the mechanisms by which antioxidants may affect immune status.