Your immune system has two approaches: the first wave is a bunch of attacks that make your body less hospitable to germs, like a fever, mucous, and achy lethargy (which keeps you at home, away from opportunistic infectious agents); the second is a tailored antibody attack that kicks in about ten days later.
Both measures require adequate levels of vitamins A, C and D — levels that you're almost certainly attaining, just by eating normal food, in which these vitamins are "practically unavoidable."
None of the alleged "immune boosters" — "echinacea, selenium, beta-carotene, green tea, bioflavonoids, garlic, and wheatgrass supplements" — have any demonstrated effect on immune response. They just give you "expensive wee."
But even if they did work, they'd just make you feel worse (see approach 1).
Here's what helps you avoid and fight illness: reducing stress (and hence cortisol levels), not getting older (younger people are better at fighting disease off), taking moderate exercise, eating a healthy diet, washing your hands frequently, keeping surfaces clean, and teaching the people around you not to sneeze on you.
"The only thing that seems to have a little bit of reasonable evidence behind it is zinc supplements to prevent colds in children," he says, adding that methodological weaknesses in these studies, too, leave him unconvinced of a real effect. "And the findings have never been replicated in adults. I certainly haven't been persuaded to recommend any immune-boosting supplements to my patients. People should be extremely careful before they part with their hard-earned money for any products that are sold to prevent the common cold or other infections."
Yet look at websites of reputable high street retailers, chemists as well as health food stores, and you'll find hundreds of products that promise to boost, tune, support or enhance your immune system. Along with the classic vitamins and minerals you'll see echinacea, selenium, beta-carotene, green tea, bioflavonoids, garlic, and wheatgrass supplements, all of which – pending any evidence that they actually work – are unlikely to do anything other than give you expensive wee. And as that's a fairly difficult thing to show off at a dinner party, it's probably not worth the investment.
Why bingeing on health foods won't boost your immune system [Dara Mohammadi/The Observer]