Our immune system is the first line of defence against illness - everything from colds to Covid.
The coronavirus outbreak has pushed our immune system centre-stage and made us realise the importance of having healthy defences.
The core of the immune system is a vast army of white blood cells, otherwise known as immune cells, which fend off against viruses, fungi and parasites that make us ill.
There are two types of immune cells, the front line defenders that are all about speed and aggression and the specialists that come in to produce custom made attacks over a longer period.
These cells are vital to us fighting the disease - but how much do we all really know about how to keep it strong?
In a BBC documentary dubbed The Truth About Boosting Your Immune System, Dr Ronx Ikharia, an emergency medicine doctor, delves into the latest science to find out what we can all do to make our immune system as healthy as possible.
Dr Ronx uncovers the unexpected things that do and don't work, the one key vitamin that can make a difference and the surprising truth that for many of us, ‘boosting’ our immune system is the last thing we should be doing.
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There are a growing number of products promising to boost our immune systems.
Almost every vitamin and mineral is claimed to have immune-strengthening powers, but do any of them actually work?
Professor Michael Heinrich, from the UCL School of Pharmacy, explains that many are beneficial as food items but have no specific therapeutic effect.
There is very little evidence that elderberry or garlic supplements do much for our immune health.
Echinacea can prevent and treat symptoms of cold and mild forms of flu, but in one sample there was unbelievably close to nothing in it.
Explaining that most people should be able to get what they need from a healthy diet, Professor Heinrich says: "Normally you shouldn't need it.
"It is certainly unlikely if you take a high dose of a supplement that it has any specific health benefits. It will simply go through your body - that's not very useful."
Most people associate Vitamin C, which helps immune cells fight infection, with oranges and other citrus fruits.
But red peppers are twice as rich, with half a sweet pepper a day giving the body what it needs.
Zinc helps the body produce new immune cells and comes in meat, cheese and a wide range of seeds and kernels.
B Vitamins, which help provide energy to immune cells, can be found in peas, fortified cereals and yoghurts.
Seafood is the real champion, as one serving of muscles contains seven times your daily recommended intake of vitamin B12.
"It doesn't harm to add certain supplements if you think it helps you," says Professor Heinrich, but he has a stark warning.
"The worry is if the diet is not adequate and rich and you try to compensate it with some sort of a supplement. That's when things go wrong and get pear-shaped."
It is almost impossible to maintain healthy levels of Vitamin D, which makes immune cells better at killing viruses and bacteria, during dark winters.
According to the professor, Vitamin D is the one supplement we should all be taking, even in the summer if you work inside for long periods.
Worrying impact of alcohol
Alcohol reduces your defences and makes you more likely to get ill.
A small bit of alcohol in moderation won't do much harm to your immune system - with a glass of red wine over dinner actually improving your circulation.
But a lot of booze is not good.
Alcohol significantly reduces the amount of lymphocytes you have, with some dropping by as much as half after a drinking session.
This makes you more prone to infection, so drinking too much alcohol makes your immune system suffer.
The good news is that it does recover over time, so having breaks between boozing does let your immune cell levels bounce back.
Surprising benefit of stress
A surprising way to bolster your immune defences is to actually get stressed.
Stress obviously has a bad reputation and is related to heart disease, diabetes and obesity.
However, the 'fight or flight' response we have to stress can actually be beneficial to the immune system.
But it's all about the duration, as long-term chronic stress you experience for a long time every day does harm, while short-term acute stress is a survival mechanism.
When the body sees something it perceives as danger it needs to prepare to respond appropriately.
Just 20 minutes after being terrified by holding a tarantula, there is a big change as Ronx's white blood cell count goes up by over 20%.
Dr Glen Davison from the University of Kent explains: "It means when you're exposed to a challenge like a pathogen or bug the immune system is more primed. It's about the speed of that response."
But you don't have to go through a stressful ordeal to get the same benefits, as a shower switched to cold at the end for just 30 seconds initiates the stress response.
Frightening yourself with a scary movie or public speaking has been shown to produce the same effect.
Exercise pros and cons
Moderate exercise is great for stimulating the immune system to help it work optimally and increase certain lymphocyte cells by six times.
When blood flow increases, immune cells move more rapidly into our veins, responding to the physical stress in our body.
Some research has found exercising five times a week can cut the duration of colds in half, but it has to be the right intensity.
"To get the benefit we're talking about moderate exercise. Getting yourself warm, heart rate going, maybe a bit of a sweat up," explains Professor Sheena Cruickshank, an immunologist at the University of Manchester.
"If you do really extreme exercise regularly, real power athlete stuff, that can make you more susceptible to upper respiratory tract infections. It's around a two-to-six-fold increase."
Regular bouts of exercise at a level where you can talk will help you get ill less often, but too much extreme exercise may leave you more vulnerable to infection.
Dangers of boosting too much
The surprising truth that for many of us is that 'boosting' our immune system is the last thing we should be doing.
If our blood is constantly flooded with aggressive cells, it means our immune system is fully active even if there is no infection to fight.
Continuing this state can mean our immune cells damage healthy tissue and make you feel fatigued.
Evidence shows we are actually more prone to illness if we remain in a heightened state as the immune system is too overworked to fight off threats.
In the UK, over 44% of people live with a condition caused by an overactive immune system - allergies.
Every allergic response is an immune overreaction, as the immune cells go into attack mode to fight off the perceived threat even though it is harmless.
While it's based on genetics, our modern lifestyles are also to blame, with the number of allergy sufferers in the UK ballooning in the last decade.
Dr Mohamed Shamji from Imperial College London says: "Diet plays a very important role. Eating junk food can make you predisposed to allergies."
Another reason is that we are exposed to fewer microbes, so the immune system is prone to overreacting when we do encounter harmless foreign entities.
Those children who grow up on farms have fewer allergies than those who live in cities.
While allergies can't be cured, you can reduce the chances of them developing in the first place by getting out in nature more.
Ronx teams up with Professor Cruickshank to run a unique experiment.
They recruit a group of volunteers and give them an ‘immune makeover’ - changing their diet, exercise and sleep habits for six weeks to test the impact on their immune health and discover how we might all benefit.
The best way to test their immune systems is to work out the neutrophil to lymphocyte ratio (NLR).
Having a lot of neutrophil, the front line white blood cells, indicates the immune system is agitated and aggressive, which is not necessarily a good thing as good cells can be damaged.
A healthy immune system should have a good balance of neutrophil and specialist lymphocyte cells.
Five out of six of the volunteers had a really high neutrophil to lymphocyte ratio, which suggests their immune system is out of kilter.
A recent study found patients with the highest NLR scores were six times more likely to have Covid illness.
While researchers found gut bacteria train immune cells to spot friend from foe, teaching them that not every germ is bad for us.
Fibre promotes growth of gut bacteria and provides vital ammunition to immune cells, allowing them to fight off infection.