While broccoli may not have been subject to the same level of coronavirus panic buying as loo paper or hand sanitiser, it's a bit scarce in supermarkets these days.

So what is it about this green cruciferous vegetable that has suddenly piqued everyone's interest?

Green vegetables, along with supplements, exercise and juicing, are often touted as immune system boosters; but as far as scientists such as Marc Pellegrini are concerned, vaccines are the only thing that will boost your immune system to prevent infection.

"[Vaccines] boost the capacity of the immune system to a fight an infection, because you're exposing it to a pathogen that it will recognise next time," said Professor Pellegrini, an infectious diseases expert at the Walter and Eliza Hall Institute.

Although there is currently no vaccine to boost your immune system and prevent COVID-19, you can try to keep your immune system — and your body — healthy to give it the best chance of fighting an infection.

This is where things like a balanced diet, regular exercise and sleep come in.

But first, let's unpack how your immune system works and why some people have better immune systems than others.

What happens when your immune system meets a virus?

Your immune system is made up of different types of cells and molecules, such as antibodies.

The first line of defence is what's known as our innate immune system.

Every cell in your body is primed to make interferons — antiviral molecules — when they detect an intruder.

"These cells will start to make their own inherent antiviral molecules that will try and stop, for example, viruses from replicating," Professor Pellegrini said.

This innate response, which kicks in immediately, produces substances called cytokines, which cause fever and inflammation of the tissues as the cells start to die.

"That's a natural mechanism for them to try and commit suicide if they recognise they've been infected."

There are also white blood cells, known as natural killer cells, which detect infected cells and kill them.

The second line of defence occurs in a spectrum of other white blood cells such as monocytes, macrophages and neutrophils that survey the environment and try and recognise infection, and release immune hormones to try and prepare other cells for the likely scenario that they might get infected.

The third line of defence is the adaptive system, which takes several days to kick in. These are white blood cells such as T-cells that try to kill infected cells, and B-cells that produce antibodies that either neutralise the bug or coat them with a substance so they can be recognised by the T-cells.

The problem with the new SARS-Cov-2 virus is that we have no antibodies or adaptive immune system.

If the immune system can't stop the virus replicating, it goes into overdrive and ramps up inflammation, especially in the lungs. This is what causes viral pneumonia.

"This is where it's critical for our body to be nimble. And the nimble part of the immune system is the innate system."

Do some of us have weaker immune systems?

The immune systems of the very old and the very young are weaker than most other people.

"As you age, some of the cells also age and become a little less nimble in their capacity to respond to the infections."

Usually babies and very young children also have a greater risk because their innate immunes system hasn't matured, but that does not appear to be happening with COVID-19.

People who are immunocompromised are also less nimble. This includes people who have a disease or are taking a drug that suppresses their immune system such as those with autoimmune conditions such as rheumatoid arthritis, cancer who are undergoing chemotherapy and people who have had organ transplants.

What can put your immune system under stress?

If you are already fighting an infection, conditions such as heart disease, lung disease and diabetes may also put more load on your body's ability to cope and function.

"It's not as if all the [serious illness from] COVID-19 we're seeing is caused by people who have got an inept immune system," Professor Pellegrini said.

"It's probably more a spectrum of people who have got subtle immunosuppression ... but that is overlaid with a lot of people who've probably got a normal immune system, but have got damaged lungs or heart that just cannot cope with the stress of having an infection."

For example, if the heart doesn't have capacity to deliver oxygen to the body it has to beat harder, which in turn can cause a heart attack.

Or if part of the lung is damaged there is less ability to get oxygen into the blood.

Having a concurrent infection, such as the flu, can also make it harder for your immune system to cope, which is why doctors are recommending you get a flu shot.

Stress may also affect your immune system, although we're not exactly sure how, Professor Pellegrini said.

"If you're under tremendous psychological stress then your adrenaline and cortisol levels will be high and these too could impact immunity."

Are some foods better for your immune system?

"Anything that makes your heart healthy, your lungs healthy, and kidneys healthy, will make your immune system healthy," Professor Pellegrini said.

There is no particular diet, other than avoiding highly processed foods loaded with sugar.

If your supermarket has run out of broccoli never fear.

"I wouldn't say that people should be eating one vegetable over another [keeping your immune system healthy is] about general nutrition."

Clare Collins, a dietitian at the University of Newcastle agreed.

"There is no one food that can magically supercharge your immune system," Professor Collins said.

"Nutrients from a range of healthy foods are needed in the biochemical pathways that are triggered as your body fights an infection."

The idea, she said, is to eat a range of foods that contain vitamins A, B, C, D and E and the minerals iron, zinc and selenium.

Good sources of vitamins Vitamin A: oily fish, egg yolks, cheese, tofu, nuts, seeds, whole grains and legumes

oily fish, egg yolks, cheese, tofu, nuts, seeds, whole grains and legumes Vitamin B6 (riboflavin): cereals, legumes, green leafy vegetables, fruit, nuts, fish, chicken and meat

cereals, legumes, green leafy vegetables, fruit, nuts, fish, chicken and meat Vitamin B9 (folate): green leafy vegetables, legumes, nuts and seeds and commercial bread-making flour

green leafy vegetables, legumes, nuts and seeds and commercial bread-making flour B12: animal products, including eggs, meat and dairy, and also in fortified soy milk (check the nutrition information panel)

animal products, including eggs, meat and dairy, and also in fortified soy milk (check the nutrition information panel) Vitamin C: oranges, lemons, limes, berries, kiwifruit, broccoli, tomatoes and capsicum

oranges, lemons, limes, berries, kiwifruit, broccoli, tomatoes and capsicum Vitamin D: mainly sunlight, but is also found in some foods such as eggs, fish, while some milks and margarine brands may be fortified with it.

mainly sunlight, but is also found in some foods such as eggs, fish, while some milks and margarine brands may be fortified with it. Vitamin E: nuts, green leafy vegetables and vegetables oils.

nuts, green leafy vegetables and vegetables oils. Iron: meat, chicken and fish. Vegetarian sources include legumes, whole grains and iron-fortified breakfast cereals

meat, chicken and fish. Vegetarian sources include legumes, whole grains and iron-fortified breakfast cereals Zinc: oysters and other seafood, meat, chicken, dried beans and nuts

oysters and other seafood, meat, chicken, dried beans and nuts Selenium: nuts, especially Brazil nuts and meat, cereals and mushrooms

These micronutrients play important roles in both the innate and adaptive immune systems in the production of different types of cells.

In addition to this, vitamin A and zinc helps to maintain the integrity of the skin and lining of vital organs and the respiratory system (which are part of the innate immune system).

Vitamin B12 and iron are also essential for the production of haemoglobin which carries oxygen in the blood.

Vitamin C, E and selenium help to control inflammation by mopping up the impact of oxidative stress produced by free radicals that pierce cell walls causing the contents to leak.

What about taking extra vitamins?

Taking additional vitamins is not necessary unless you have been diagnosed by your doctor with a specific nutrient deficiency such as vitamin D, or you have specific dietary needs because you are pregnant or have been diagnosed with a condition that can affect the absorption of nutrients such as cystic fibrosis or gut disorders.

"The only people who need a vitamin or mineral supplement are those who would have already been advised but their doctor or dietitian to take one," Professor Collins said.

There is no conclusive evidence that vitamin C supplements can delay the onset of an infection or treat respiratory infections.

How does exercising affect my immune system?

Many studies show that long-term exercise is beneficial not only for the immune system but also keeping your heart and lungs healthy.

There are many ways to continue an exercise program even when you are physically distancing.

But take it slowly if you're not accustomed to exercising, Professor Pellegrini said.

"Don't suddenly try to run a marathon thinking that you suddenly become fit and resilient," he said.

"You'll be more susceptible to infection because you're diverted a huge amount of energy into building muscle and fitness — energy that is useful to the immune system."

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