Walk through the aisles of any health food shop and you’ll see pots of echinacea or zinc that promise to “support your immune system” or “maintain its healthy function”. Read new age health blogging sites and you’ll find posts on how drinking hot lemon water or knocking back a shot of wheatgrass juice or the current green goo du jour will “boost your immune system” and make you less likely to get ill. These are tempting prospects at this time of year, but ones that are foiled by an inconvenient truth: they don’t work. The idea that any dietary supplement can boost your immunity makes very little scientific sense. And because of the way your immune system works, even if they did what they say they did, you definitely wouldn’t want them to.

“People have this idea that the immune system is some kind of internal force field that can be boosted or patched up,” says Charles Bangham, a professor of immunology and infectious diseases at Imperial College London. “This couldn’t be further from the truth. As the name suggests it’s not a single thing but a system incorporating many organs and biological functions.”

The immune system can be broadly split into two parts, the innate and the acquired response. On detection of infection, it’s the innate response that acts first. Though fast, it lacks in finesse, and deals with an invading pathogen in much the same way that the Ghostbusters might try to remove a ghost from a haunted hotel. It gunges the halls and doorways to try to flush it out (that’s why you fill up with phlegm and snot), it yanks up the thermostat to try to boil it (why you run a fever), and it shuts down the building until the problem is solved (it makes you depressed and lethargic so you don’t go out and pick up another infection while your immune system is at work).

If a supplement stimulated the innate immune response it would leave you with a constant feeling of being unwell Prof Charles Bangham

What it doesn’t do is eliminate the intruder from the body. That’s the remit of the acquired system, a well-drilled Swat team by comparison that identifies the enemy and makes the specific weapons, or antibodies, needed to destroy it.

“You might ask why we’ve kept this evolutionarily primitive innate system that makes us feel ill if we’ve all got perfectly good acquired systems,” says Bangham. “It’s because it takes around five to 10 days for the acquired system to identify the antibodies needed and clone them up to sufficient numbers to make a meaningful attack. The immune system is there to curb the pathogen’s multiplication in that window of opportunity before it finally gets clobbered.”

There is of course a way you can speed up this process and thereby boost your immunity. It’s called vaccination. Vaccines contain harmless versions of the bug you want to protect yourself against so that the acquired system can remember them and act more quickly the next time it meets them. Given that the supplements you buy in health food shops are not vaccines and thus can’t be helping the acquired system, they must therefore be aiming to boost the innate one, which would be far from ideal.

“If a supplement actually stimulated the innate immune response then it would leave you with a constant feeling of being unwell,” says Bangham, “with a fever, a snotty nose, depression and lethargy without any obvious benefit. So in that respect, the whole idea is a bit of a con.”

If you have a normal diet, you don’t need supplemental vitamins or minerals. Photograph: Alamy

Like most pseudoscientific concepts, immune-boosting supplementation has a grounding in misunderstood science. Vitamins, especially A, C and D, and minerals such as zinc do have a vital role in the functioning of our immune system, but they are also practically unavoidable components of our diets, present in large quantities in fruits, vegetables and meats. If you have a normal diet, you don’t need supplemental vitamins or minerals, and giving more of them won’t help.

“They [manufacturers of supplements] might not say anything untrue,” Bangham says “but what they are doing is implying that if someone on a normal diet takes them they will improve their immune function, which is plain wrong.”

Most of the studies that apparently support the effectiveness of such supplements, says Tim Ballard, vice-chair of the Royal College of General Practitioners, are done in selected groups of patients with specific deficiencies and are incorrectly extrapolated to the general population.

“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.

‘There’s increasing evidence that you actually want people to come across allergens and develop colds,’ says Tim Ballard, vice-chair of the Royal College of General Practitioners. Photograph: Alamy

So what can we do to avoid getting ill? “Other than a healthy diet and regular light exercise, which increases the activity of helpful immune cells, simple personal hygiene is important,” says Ballard. “I’m not suggesting that people are pathologically dirty but there’s evidence that washing your hands, being careful not to sneeze over people, keeping surfaces nice and clean all reduce the risk of transmission.”

So, basically, everything that your mum told you to do. Though we should stop short of becoming antibacterial wipe-wielding germaphobes. “There’s increasing evidence that you actually want people to come across allergens and develop colds, especially children,” he says, “it’s all part of the normal maturation of the immune system.”

There is still much about the immune system that researchers have to discover. In only the past decade, with genetic sequencing technologies becoming more affordable, scientists have begun to learn not only how invading pathogens shape and strengthen our immune system, but also about how it interacts with the communities of microbes that live in and on our bodies.

“Research around the microbiome is fascinating,” says Ballard of attempts to better understand the interplay between our immune system and the trillions of microbes that line our skin, gut and lungs, also called our microbiota. “A lot of people don’t realise that just over half the cells in our body are not human but are actually bacteria and other micro-organisms. Certainly to be in good health you need a good balance between people and their various microbes.”

Liam O’Mahony is a molecular immunologist at the Swiss Institute of Allergy and Asthma Research in Davos. He wants to find out how to tweak people’s microbiota to improve their resistance to allergic and infectious diseases. It’s a line of research that might one day fulfil the empty promises made by immune-boosting supplements.

The microbes in and on our body, he says, help defend us from pathogenic infection on several levels. The first is colonisation resistance. “By just being there they’re taking up real estate, if you like, competing for space and food so that disease-causing bugs can’t establish,” he says.

The second is by secreting antimicrobial molecules that kill potentially dangerous pathogens. Bacteria and fungi secrete toxic proteins to fend off undesirable species; it’s an evolutionary arms race that we’ve latched on to by extracting these proteins and using versions of them as antibiotic drugs.

The third is by regulating the inflammatory signals of our innate immune response and thus the feelings of sickness we get when that innate system jumps into action. “The different composition of people’s microbiotas might explain why although we’re all exposed to the same bugs, some of us have only mild symptoms from infections and other people get knocked down by them,” he says.

O’Mahony hopes that by better understanding what an ideal microbiota is we can boost some people’s natural immunity to infection. That dream is not too far away – it’s already happening in the clinic, albeit in a very basic form. In what sounds like something from a Roald Dahl recipe book for disgusting medical concoctions, the use of faecal transplants – taking the poo from a healthy individual and transplanting it, along with all its friendly bacteria, to the gut of a patient through their bottom or nose – is gaining popularity.

“People are doing faecal transplants and getting incredible cure rates for Clostridium difficile infections,” he says. These infections occur when a strong course of antibiotics kills the friendly bacteria in a patient’s gut allowing the potentially deadly and highly drug-resistant C difficile bacteria to land-grab and colonise the gut. “Faecal transplants obviously don’t sound too appetising and the application is limited,” he says, “so what would be better is if we could define what the ideal five to six hundred bacteria for an individual are and grow them in a lab and give them to people as personalised medicine.”

If he and others working in the field can figure out what an ideal microbiota looks like for different people, they can then supplement or patch up an individual’s bacteria, with say a spray or a cream, to make them more resistant to infections. “The technology is there from a laboratory point of view,” he says, “but I think it’ll be another few years before it’s ready to start clinical testing and a few years on from that before anything hits the market.”

Until that point, unless you fancy a DIY faecal transplant, if you want to maintain your immunity and stay in good health it’s best to eat well, do some exercise, and save your pennies.

Regular exercise helps maintain a healthy immune system. Photograph: Alamy

So what does affect the immune system?

Cold

The thing about old wives’ tales is that many people believe them. Take the widely held but incorrect assumption that wrapping up warm will protect you from catching a cold. The common cold does have seasonal spikes in colder months, but nobody knows exactly why. The most likely answer is that in winter we spend most of our time huddled inside in the warmth with other people and all their bugs. But don’t throw away your hat or scarf just yet – keeping warm will keep you more comfortable and protect you from things like frostbite.

Stress

Your angry boss may be why you’re spending more time off work, but not for the reason you may think. “Long-standing psychosocial stresses like those in the workplace or from an unhappy marriage can reduce your immune function and leave you more prone to infection,” says Professor Janet Lord, an immunologist from the University of Birmingham. “Chronic stress produces a stress hormone called cortisol that kills or neutralises your immune cells.” Other than your preferred method of winding down, she says, exercise will help, as it reduces the levels of stress hormones.

Age

“Our immune systems get weaker as we get older,” says Professor Arne Akbar from UCL who, like Professor Lord, is a spokesperson for the British Society for Immunology. “We know this because as we age we see more infections that were previously controlled, like chicken pox, which returns in old age as shingles; and because vaccines don’t work quite as well in older people as they do in children.” After decades of activity and proliferation of immune cells, organs in the immune system such as the thymus begin to tire and to stop producing new cells. “We lose cells and progressively lose ‘memories’ of how to deal with pathogens,” he says. “Our body gets worn down by this constant dance towards death.”

Exercise

Getting up on your feet will boost your immunity. While the exact threshold for benefit is hard to establish, improving your circulation can give your immune cells a better chance of being where they need to be. “When your heart beats more, your immune cells that patrol for problems move around more,” says Professor Akbar. “By exercising you keep them out looking for potential problems rather than sitting around waiting for problems to happen.”