pillar  09

Immune Health and
Disease Resistance

Why Your Body’s Defence System Works Better Than You Think — and How to Keep It That Way
Pillar 09 of 12 | Reading Time: 18 minutes
 
 
 
Content last reviewed: January 2026.
Based on peer-reviewed research available at time of publication. Medical science advances continuously. Always consult a qualified healthcare provider for personalised medical advice.

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the defence system working rright now

While you are reading this, your immune system is working.
Not resting.
Not waiting.
Working.
Right now.
It is identifying and neutralising, which means finding and destroying, bacteria that entered your body with your last meal.
It is monitoring cells throughout your body for signs of abnormal growth.
It is patrolling your bloodstream for viruses that entered through the air you just breathed.
It is managing inflammation, which is the body’s natural response to injury and infection, in tissues recovering from yesterday’s exercise.
It never stops.
Not for a single hour.
Not for a single minute.
And whether it does all of this well or poorly depends almost entirely on how you live.
 
Most people only think about their immune system when it fails them.
When they catch a cold that will not go away.
When they seem to get ill more often than other people.
When they recover more slowly than they used to.
But by then the immune system has already been quietly compromised, which means gradually weakened, for months or years.
By poor sleep.
By chronic stress, which means stress that continues for a long time without relief.
By insufficient movement.
By a diet that does not give it what it needs.

 

This pillar is about understanding how your immune system actually works and what it needs to work well.
Not to make it overactive, which would cause it to attack the body’s own tissues.
Not to boost it beyond its natural limits.
Simply to give it the conditions it needs to do the remarkable job it was designed to do.

a story about two neighbours

Picture two women who live next door to each other.
Same street.
Same climate.
Same exposure to the same viruses and bacteria every single day.
Rachel is 43.
She gets ill four or five times a year.

 

Each illness lingers longer than it should.
She recovered from a chest infection two months ago and her energy has still not fully returned.
She is on her second course of antibiotics, which are medicines used to treat bacterial infections, this year.
She is tired more days than not.
Her wounds heal slowly.
She has dark circles under her eyes that never quite disappear.
She has accepted this as her normal.

 

Her neighbour Beth is also 43.
She gets ill perhaps once a year.
When she does, she recovers in a few days.
She has not needed antibiotics in three years.
Her energy is consistent.
Her wounds heal cleanly.
She sleeps deeply and wakes feeling rested.

 

Rachel and Beth are not inherently different people.
They are not dealing with fundamentally different immune systems.
But they are living in fundamentally different ways.
And their immune systems are performing accordingly.

what the immune system actually is

The immune system is not a single organ.
It is a vast, distributed network spread throughout the entire body.
Operating simultaneously in many places at once.
Think of it like a national defence network.
With different branches responsible for different threats.
Different types of weapons for different types of enemies.
Intelligence units that remember past invaders.
Rapid response teams for immediate threats.
And constant communication between all units to coordinate the response.

The key players include white blood cells, which are a type of blood cell whose job is to detect and destroy threats.
White blood cells come in many different types, each with a specific role.
The lymphatic system, which is a network of vessels, which are like tubes, and nodes, which are small glands, that carries immune cells around the body.
Think of the lymphatic system as the road network that immune cells travel along.
The spleen, which is an organ in the upper left of the abdomen, meaning the belly area, that filters the blood and removes damaged or infected cells.
The thymus gland, which is a small gland in the chest where certain immune cells are trained and matured before being sent to work.
The bone marrow, which is the soft tissue inside bones where most immune cells are produced.
And crucially, the gut, where approximately 70 per cent of immune tissue lives and where the gut microbiome, which is the community of microorganisms living in the digestive system, trains immune cells throughout life.

All of these systems communicate and coordinate continuously.
The World Health Organisation describes the immune system as the body’s primary defence against infectious disease, abnormal cell growth, and foreign substances.

the two branches of immunity

The immune system has two main branches.
They work together but they operate differently.

The Innate Immune System:

This is your first line of defence.
It responds immediately.
The moment a pathogen enters the body, the innate immune system is activated.
A pathogen is any organism, meaning any living thing, that causes disease.
This includes bacteria, viruses, fungi, and parasites, which are organisms that live inside other organisms and cause harm.
Think of the innate immune system as the border patrol.
It is always stationed at the boundary between you and the outside world.
It does not need time to identify the specific threat.
It simply recognises that something foreign and potentially harmful has crossed the line.
And it responds immediately.
Inflammation is one of the primary weapons of the innate immune system.
The heat, redness, swelling, and pain you see around a cut or infection are signs that the innate immune system is actively fighting.

The Adaptive Immune System:

This is your second line of defence.
It is slower to activate but far more precise.
The adaptive immune system studies the specific invader and designs a targeted response.
Targeted means aimed precisely at the specific threat.
Think of it like a specialist unit that is called in after the border patrol has detected the threat.
The specialists analyse the enemy in detail.
They design the perfect weapon to defeat that specific invader.
And then they do something remarkable.
They remember.
Every pathogen the adaptive immune system defeats is recorded in what scientists call immunological memory.
Immunological memory simply means the immune system’s ability to remember past invaders and respond faster the next time.
The next time that same pathogen enters the body, the adaptive immune system recognises it immediately.
It deploys the perfect response before the invader can establish itself.
This memory is the principle behind vaccination.
Vaccines introduce a harmless version of a pathogen, or a fragment of one, to the body.
The adaptive immune system studies it.
Creates the memory.
And is ready to respond rapidly if the real pathogen ever arrives.

what inflammation actually means

Inflammation is one of the most misunderstood terms in health.
Most people think of it as entirely negative.
Something to be eliminated.
But acute inflammation, which means inflammation that lasts for a short time in response to a specific event, is not the enemy.
It is essential.
When you cut your finger, the area becomes red, warm, and slightly swollen.
That is acute inflammation.
Think of it as the emergency repair crew arriving at a damaged site.
They section off the area.
They bring in the equipment.
They begin repairs.
The redness and swelling are signs that repair is underway.
Without this response, wounds would not heal.
Infections would spread unchecked.
The body could not recover from injury.
Acute inflammation is the immune system doing exactly what it should.
 
The problem is chronic inflammation.
Chronic means long-lasting.
Chronic inflammation is when the emergency repair crew never leaves.
The barriers stay up.
The equipment keeps running.
Week after week.
Month after month.
Year after year.
Chronic low-grade inflammation, which means mild but persistent inflammation throughout the body, damages blood vessel walls, contributes to insulin resistance, which means the body becoming less responsive to the hormone that manages blood sugar, promotes abnormal cell growth, and accelerates brain ageing.
It is now understood to be a driving force behind heart disease, type 2 diabetes, Alzheimer’s disease, and certain cancers.
Research published in Nature Medicine describes chronic low-grade inflammation as one of the primary biological drivers of ageing and age-related disease.
The causes of chronic inflammation are precisely the things this entire website addresses.
Poor diet.
Poor sleep.
Physical inactivity.
Chronic stress.
Obesity, which means carrying significantly more body fat than is healthy.
Smoking.
Alcohol.
These are not separate problems.
They are the same problem viewed from different angles.

what weakens the immune system

Poor sleep:

 

Sleep is when the immune system does its most intensive work.
During deep sleep, the body produces cytokines.
Cytokines are tiny proteins, which are molecules made of amino acids, that coordinate the immune response.
Think of cytokines as the dispatchers who send immune cells to where they are needed most.
Without adequate sleep, the dispatch centre is understaffed.
The response is slower.
The coverage is thinner.
Research published in Sleep found that people who slept fewer than seven hours were almost three times more likely to develop a cold after being exposed to a cold virus than those who slept eight or more hours.
Three times more likely.
From one variable.
Sleep.

 

Chronic stress:

 

When cortisol, which is the body’s primary stress hormone, is elevated for extended periods, it actively suppresses, which means reduces the effectiveness of, immune function.
Research published in Psychosomatic Medicine found that people under significant psychological stress were more than twice as likely to develop a cold after being directly exposed to cold viruses.

 

Poor nutrition:

 

The immune system requires specific micronutrients, which are vitamins and minerals needed in small amounts, to function properly.
Vitamin D, which the skin produces in response to sunlight, activates the immune cells that detect and destroy pathogens.
Without adequate vitamin D, these cells cannot fully activate.
Think of vitamin D as the ignition key for the immune activation system.
Without the key, the engine cannot start.
Zinc, which is a mineral found in meat, shellfish, seeds, and legumes, is essential for the production of immune cells.
Without it, the production line slows.
Vitamin C, which is found in citrus fruits, peppers, broccoli, and kiwi, supports the function of multiple immune cells and acts as a powerful antioxidant.
An antioxidant is a substance that protects cells from damage caused by reactive molecules called free radicals.
Free radicals are unstable molecules produced naturally during the body’s chemical processes.
Think of free radicals as sparks from a fire.
In small amounts they are manageable.
In large amounts they damage surrounding tissues.
Antioxidants neutralise them, which means they make them harmless.

 

Physical inactivity:

 

Regular moderate exercise is one of the most consistent predictors of healthy immune function.
Moderate means not too intense and not too light.
Movement circulates immune cells more efficiently through the lymphatic system, which is the network of vessels that carries immune cells around the body.
It reduces chronic inflammation.
It supports the health of the gut microbiome, where 70 per cent of immune tissue lives.
Physical inactivity allows chronic inflammation to persist and immune circulation to stagnate, which means slow down and become less effective.

 

Smoking and heavy alcohol consumption:

 

Both directly impair, which means reduce the effectiveness of, immune function.
Smoking damages the cilia in the respiratory tract.
Cilia are the tiny hair-like structures that line the airways.
Think of them as tiny brushes sweeping pathogens and debris out of the airways.
Smoking destroys these brushes.
The airways become more vulnerable to infection.
Heavy alcohol impairs the production of immune cells, damages the gut lining, and disrupts the gut microbiome that houses most immune tissue.

what sleep does for immunity

Sleep is not just important for immunity.
It is essential in ways that cannot be replaced by anything else.
During deep sleep, the immune system runs its most intensive operations.
It produces and releases cytokines, which are the tiny proteins that coordinate the immune response.
It activates the T cells that identify and destroy infected cells.
 
T cells are a type of white blood cell, which is an immune cell, that are central to the adaptive immune response, which is the targeted arm of the immune system that creates specific weapons against specific threats.
Think of T cells as the specialist soldiers of the immune system.
The ones trained to recognise and eliminate specific threats.
Research published in the Journal of Experimental Medicine found that sleep promotes the ability of T cells to attach to and destroy their targets.

 

During sleep, levels of certain stress hormones drop.
These stress hormones, when elevated, actively interfere with the ability of T cells to function.
Sleep removes that interference.
T cells work more effectively.
The immune response is stronger.
 
The NHS notes that poor sleep is consistently linked to increased susceptibility to illness, which means being more likely to get sick, and slower recovery.
Protecting sleep is protecting immunity.
Every night.

what nutrition does for immunity

Vitamin D:


Vitamin D is arguably the most important micronutrient for immune function.
It activates the immune cells that seek out and destroy bacteria and viruses.
Without it, this activation is impaired.
Think of vitamin D as the ignition key for the immune activation system.

The NHS recommends that adults in the UK consider taking 10 micrograms of vitamin D daily during autumn and winter when sunlight is insufficient.
A microgram is a unit of measurement equal to one millionth of a gram, an extremely tiny amount.
Deficiency, which means not having enough, is extremely common in countries far from the equator.
If you live far from the equator, supplementation, which means taking the vitamin as a supplement or tablet, during the darker months is a simple and impactful immune investment.

Zinc:


Zinc is a mineral that is required for the development and function of immune cells throughout the body.
A mild zinc deficiency, which means not having quite enough, impairs both the innate immune response, which is the immediate first line of defence, and the adaptive immune response, which is the targeted second line.
Good food sources include meat, shellfish, legumes, which are foods like lentils, beans, and chickpeas, seeds, and nuts.

Vitamin C:


Vitamin C supports the function of multiple immune cells.
It enhances the production of white blood cells, which are the cells whose job is to detect and destroy threats.
It protects immune cells from oxidative damage, which is the damage caused by free radicals, the unstable molecules that can harm cells.
The best food sources are citrus fruits like oranges and lemons, kiwi, red and yellow peppers, broccoli, and strawberries.

Omega-3 Fatty Acids:


Omega-3 fatty acids are a type of healthy fat found in oily fish like salmon, mackerel, and sardines, as well as walnuts and flaxseeds.
These fats reduce chronic inflammation throughout the body.
They support the resolution of acute inflammation, which means they help the immune response wind down once the threat has been dealt with.
Think of omega-3s as the signal that tells the emergency repair crew the work is done and it is time to leave.
Without sufficient omega-3s, the crew stays longer than it should.

Fibre:


Dietary fibre, which is the indigestible part of plant foods, feeds the beneficial bacteria living in the gut.
The gut is home to 70 per cent of immune tissue.
A well-fed, diverse gut microbiome means a better-trained, better-regulated immune system.
A diet low in fibre underfeeds the immune system’s primary training ground.

what movement does for immunity

Regular moderate physical activity is one of the most consistent and well-documented lifestyle behaviours for immune health.

Moderate means not extreme, just gently raising the heart rate.
And the effects are direct.
When you move, blood and lymph, which is the fluid that carries immune cells through the lymphatic system, circulate more efficiently.
Immune cells that travel through the lymphatic system reach tissues faster.
Think of moderate exercise as turning on the circulation pumps for the immune system.
Research published in the British Journal of Sports Medicine found that people who exercised regularly had significantly fewer upper respiratory tract infections, which are infections of the nose, throat, and airways, than those who were sedentary, which means barely moving.
And when active people did get ill, their symptoms were milder and their recovery was faster.
Regular movement also reduces chronic inflammation over time.
It supports a healthy gut microbiome.
It improves sleep quality.
All three of these benefits directly support immune function.

The important word is moderate.
Extreme, prolonged exercise without adequate recovery can temporarily suppress immune function.
This is sometimes called the open window of immunosuppression.
Immunosuppression means a reduction in the effectiveness of the immune system.
Think of it like leaving a door open after a very long, exhausting journey.
For a few hours after extreme exertion, which means extreme physical effort, the immune response is slightly reduced.

This is why elite athletes sometimes experience increased illness during periods of very high training.
For the vast majority of people, this is not a concern.
Moderate daily movement consistently strengthens rather than suppresses immunity.

what stress does for immunity

The connection between stress and immune function is one of the most robustly documented findings in all of health science.

Robustly documented means supported by a large amount of strong evidence.
Cortisol, which is the primary stress hormone, is an immunosuppressant, which means it suppresses, or reduces the effectiveness of, the immune system.
In short bursts it is not a problem.

In fact, brief cortisol surges can briefly enhance, which means temporarily improve, certain aspects of immune response.
But when cortisol is chronically elevated, which means permanently too high through ongoing stress, it suppresses the production and function of immune cells across the board, meaning throughout the entire immune system.
Think of chronic cortisol as a slow leak in the immune system.
Not a catastrophic failure.
Just a persistent drain on its effectiveness.
Over time the immune system is running with less than it should have.
Less than it was designed to have.
The result is getting ill more often.
Recovering more slowly.
Being more susceptible, which means more easily affected, by the viruses and bacteria that are always present in the environment.

Research published in Psychosomatic Medicine found that people under significant psychological stress were significantly more likely to develop a respiratory infection, which is an infection of the breathing system, after viral exposure than those who were not stressed.
Managing stress is immune management.
They are the same thing.

how to build resilient immune system

Resilient means strong enough to recover quickly from difficulties.

Sleep seven to nine hours every night:


The immune system does its most intensive work during sleep.
Protecting sleep is protecting immunity.
There is no supplement, which is a tablet or product, that replaces what deep sleep does for immune function.

Move your body every day:


Moderate daily movement circulates immune cells, reduces chronic inflammation, and supports gut microbiome health.
21 minutes per day is enough to produce meaningful benefit.

Eat a wide variety of colourful plant foods:


Different colours of vegetables and fruits contain different vitamins, minerals, and antioxidants, which are substances that protect cells from damage.
Together they provide the nutritional foundation the immune system needs.
Aiming for 30 different plant foods per week supports the gut microbiome that houses the majority of immune tissue.

Supplement vitamin D during the darker months if you live far from the equator:


The equator is the imaginary line around the middle of the earth.
Countries far from the equator receive less sunlight, especially in winter.
Less sunlight means less vitamin D is produced in the skin.
Supplementation during these months is one of the most evidence-based immune investments available.
Talk to your doctor about appropriate dosing, which means the right amount, for your situation.

Manage stress actively:


Lower cortisol means a stronger immune system.
Every tool for stress management is a tool for immunity.

Do not smoke:


Smoking destroys the cilia, which are the tiny brushes lining the airways that sweep pathogens out.
It impairs immune cell function throughout the body.
There is no amount of smoking that supports immune health.

Drink alcohol moderately at most:


Heavy alcohol directly suppresses immune function.
Moderating, which means reducing, or eliminating alcohol is a direct immune investment.

Spend time outdoors:


Sunlight on skin produces vitamin D.
Time in nature reduces cortisol.
Both directly support immune function.

key takeaways

✓ The immune system never stops working. It is defending your body every second of every day. Whether it does this well depends on how you live.

✓ People who slept fewer than seven hours were almost three times more likely to develop a cold after viral exposure than those sleeping eight or more hours according to research published in Sleep.

✓ People under significant psychological stress were significantly more likely to develop a respiratory infection after viral exposure according to research in Psychosomatic Medicine.

✓ Sleep promotes the ability of T cells, which are specialist immune cells, to attach to and destroy their targets according to research in the Journal of Experimental Medicine.

✓ Approximately 70 per cent of immune tissue lives in or around the gut. Gut microbiome health and immune health are inseparable.

✓ Chronic low-grade inflammation, which means mild but persistent inflammation throughout the body, is one of the primary biological drivers of ageing and age-related disease according to research in Nature Medicine.

✓ Regular exercisers had significantly fewer respiratory infections and milder symptoms when they did get ill according to research in the British Journal of Sports Medicine.

✓ Vitamin D activates the immune cells that detect and destroy pathogens. The NHS recommends supplementation during darker months in countries far from the equator.

✓ Sleep, movement, plant-rich nutrition, vitamin D, stress management, and not smoking are the six most impactful immune investments available to everyone.

frequently asked questions

Why do I keep getting ill more often than other people?
Frequent illness almost always reflects one or more of the following.

Consistently poor sleep.
Chronic high stress, meaning stress that continues for a long time without relief.
A diet low in the micronutrients, which are vitamins and minerals, the immune system needs.
Physical inactivity.
Smoking or heavy alcohol use.
A disrupted gut microbiome from poor diet or frequent antibiotic use.
Identifying which of these applies and addressing it consistently is almost always more effective than any supplement or quick fix.
The idea of boosting the immune system is popular but scientifically imprecise.
Boost means to increase beyond normal.
An overactive immune system is not healthy.
It produces autoimmune conditions, which are conditions where the immune system attacks the body’s own tissues.
 
The goal is not to boost immunity but to support it.
To give it the conditions it needs to function at its designed capacity.
Sleep, nutrition, movement, and stress management achieve this.
No supplement or product does it more effectively.
Vitamin C is important for immune function.
But the evidence that supplementing, which means taking extra vitamin C as a tablet, prevents colds in the general population is modest.

Modest means the effect is real but not large.
Where vitamin C supplementation shows stronger benefit is in people under significant physical stress, such as endurance athletes.
Endurance athletes are people who train and compete in activities requiring prolonged physical effort like long-distance running.

For most people, eating a varied diet rich in fruits and vegetables provides adequate vitamin C and is more beneficial than supplementation alone.
Both, depending on the amount.

Moderate regular exercise, which means not too intense and not too light, consistently strengthens immune function.
It circulates immune cells more effectively and reduces chronic inflammation.

Extreme, prolonged exercise without adequate recovery can temporarily suppress, which means reduce the effectiveness of, immune function in the hours immediately after.
For the vast majority of people, regular moderate exercise is one of the most powerful immune investments available.
Extremely important.

Approximately 70 per cent of immune tissue lives in or around the gut.
The gut microbiome, which is the community of microorganisms living in the digestive system, trains immune cells throughout life, regulates the balance between inflammation and anti-inflammation, and is the primary interface, which means the main point of contact, between the outside world and the immune system.

A diverse, healthy microbiome, supported by a wide variety of plant foods and fermented foods, is one of the most powerful foundations for immune resilience.
Resilience means the ability to resist illness and recover quickly when it does occur.

medical reference

World Health Organisation. (2023). Immunization Coverage.
 
Cohen S et al. (2009). Sleep Habits and Susceptibility to the Common Cold. Archives of Internal Medicine. 169(1):62-67.
 
Cohen S et al. (1991). Psychological Stress and Susceptibility to the Common Cold. New England Journal of Medicine. 325(9):606-612.
 
Luciana B et al. (2019). Sleep Facilitates the Ability of T Cells to Recognise Antigen. Journal of Experimental Medicine. 216(3):507-526.
 
Furman D et al. (2019). Chronic Inflammation in the Aetiology of Disease Across the Life Span. Nature Medicine. 25(12):1822-1832.
 
Nieman DC and Wentz LM. (2019). The Compelling Link Between Physical Activity and the Body’s Defence System. Journal of Sport and Health Science. 8(3):201-217.
 
National Health Service. (2023). Vitamin D.
 
National Health Service. (2023). How to Get a Good Night’s Sleep.
 
Calder PC. (2020). Nutrition, Immunity and COVID-19. BMJ Nutrition, Prevention and Health. 3(1):74-92.
 
Gleeson M et al. (2011). The Anti-Inflammatory Effects of Exercise: Mechanisms and Implications for the Prevention and Treatment of Disease. Nature Reviews Immunology. 11(9):607-615.