An introduction to the immune system

Made up of a series of cells, tissues and organs, your immune system is pretty remarkable^{1, 2}.

The primary job of your immune system is to help keep you healthy by fighting off infections and anything that might be harmful to your body, no matter how small or minor³. The most amazing part? Even though your immune system is working around the clock, you probably won’t even notice, as it gets on with protecting your body with no complaints and free from drama!

So, we know it’s important, but how does the immune system work? How can you spot any immune system health problems when it stops working? And just how can we support our immune system?

The immune system is very busy, and has several essential functions¹:

• Fighting bacteria, viruses, parasites or fungi (also known as ‘pathogens’).
• Recognising harmful substances and making a log of them, which helps to fight them more effectively next time.
• Fighting any disease-causing changes that take place in your body. 

In addition to the above, the immune system cleverly helps your body heal from wounds⁴.

When the body encounters something it doesn’t recognise (some of these are known as ‘antigens’), your immune system is activated, triggering a whole host of chemical and hormonal processes¹.

If it detects something harmful, your immune system gets to work releasing white blood cells into your bloodstream and sounds the alarm to alert the rest of the immune system³. When a white blood cell senses something that could harm your body or make you unwell, it releases proteins called ‘cytokines’. Think of these as your immune system’s messengers, which efficiently communicate with other white blood cells, preparing them to attack and to protect you⁵.

Your immune system is made up of two very important parts: the innate and the adaptive.

The innate immune system is the part you are born with and is your first line of defence against any harmful germs, viruses or bacteria. It comprises your skin, mucous membranes, proteins and special immune ‘defence’ cells. In addition, tears, sweat, and stomach acids are good examples of chemical barriers to anything that might be harmful to your body⁶.

If anything harmful gets through, your body’s defence cells immediately get to work (1).  For example, if you get a paper cut or a graze, your innate immune system will respond to any bacteria that enter through the wound and deal with them as quickly as possible to protect you from infection¹.

Sometimes, things get past the innate immune system; if this happens, the adaptive immune system takes over.

While its response is slower, it’s also very clever. The adaptive immune system uses antibodies produced by memory immune cells to identify and, crucially, remember any harmful intruders. This means that should they return at any point, your immune system will know exactly what to do¹.  

If you’ve ever wondered why you only get some illnesses once and then become immune to them (chickenpox is a great example), this is your adaptive immune system at work⁷.

Immune system health problems can occur when the immune system isn't working correctly or has been weakened. If this happens, your body may be unable to fight germs effectively, making you feel unwell¹.

Several immune system disorders exist, including food allergies, hay fever, and lupus⁸. 

Signs your immune system may be weakened or compromised include⁹:

• Experiencing recurrent infections (in other words, you get the same illness over and over again)
• You come down with more severe infections on a regular basis
• You’ve become resistant to antibiotics
• You regularly suffer from colds or cold sores
• Wounds are slow to heal
• You feel fatigued and tired most of the time

If you have any concerns about your immune system, make sure you speak to your GP, who can give you the appropriate advice and guidance.

You can do plenty of things to proactively support your immune system.

Sleep is a real boost for the immune system. When you’re asleep, your immune system is still at work producing and replenishing the antibodies and proteins that form part of your body’s immune response. If you’re injured or not feeling too well, this process can help your body to repair and fight illness and infection⁵.

That’s why a good night’s sleep is crucial.

On average, adults need 7 - 9 hours of sleep per night, while children require 9 - 13¹⁰.

A varied and well-balanced diet is also essential for supporting your immune system. This is because it provides the energy and nutrients your body requires daily to keep you healthy. These include vitamins B6 and D, zinc, selenium, and iron¹¹.

• Vitamin B6. You’ll find Vitamin B6 in foods like white meat (pork, poultry), fish, peanuts, soya beans, oats, bananas and milk¹². 
• Vitamin D. Your body makes vitamin D when exposed to sunlight, and it’s also found in foods like oily fish – such as salmon, sardines, trout, herring or mackerel, red meat and egg yolks¹³. It’s important to increase dietary sources of vitamin D during autumn and winter, when daylight hours are reduced.
• Selenium, which is found in Brazil nuts, fish, meat and eggs¹⁴.
• Zinc, found in meat, shellfish, dairy foods (such as cheese) and bread¹⁴.
• Iron. Iron-rich sources include dark green vegetables, dark red meat, beans (red kidney beans, edamame beans and chickpeas), nuts and dried fruits¹⁵.
• Protein. Found in meat, dairy, legumes and nuts¹⁶.
• Antioxidants. Found in berries leafy greens and nuts.
   

You can also try Actimel yogurt shots containing vitamins D and B6, which are known to help support the functioning of the immune system.

Read more about foods that will help your immune system function.

Along with a good night’s sleep and a well-balanced diet, regular moderate exercise is also recommended to help boost your immune system—aim for at least 150 minutes of exercise across the week through various activities and try to do some form of movement every day – like walking¹⁷.

Your immune system has a delicately balanced relationship with stress. Short-term stress (for example, if you’re faced with immediate danger), causes your body to release hormones like cortisol and adrenaline, which can help your innate immune system (your first line of defence) to kick in and help¹⁸.

However, exposure to long-term stress can have the opposite effect, suppressing your immune system, causing your body to release cortisol, and interfering with digestion¹⁹.

Ultimately, there are a number of things you can do to support your immune system and keep it healthy. Why not try:
 

• Setting yourself a sleep schedule to get at least 7 hours a night.  
• Eating a healthy, balanced diet and planning your meals to include immune-support foods.  
• Incorporating exercise, such as a daily walk, into your routine to get your body moving. 

1. InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. In brief: How does the immune system work? [Updated 2023 Jun 6]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279364/
2. Justiz Vaillant AA, Sabir S, Jan A. Physiology, Immune Response. [Updated 2024 Jul 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539801/
3. NHS. Addison's disease: Causes [online 2021]. Available at https://www.nhs.uk/conditions/addisons-disease/causes/#:~:text=The%20immune%20system%20is%20your%20body's%20defence%20against%20infection%20and%20disease. [Accessed June 2025]
4. NHS Health Research Authority. The Control of Innate Immunity in disease [online 2018]. Available at https://www.hra.nhs.uk/planning-and-improving-research/application-summaries/research-summaries/the-control-of-innate-immunity-in-disease/#:~:text=The%20immune%20system%20is%20essential,diseases%20like%20asthma%20or%20arthritis. [Accessed June 2025]
5. Ferreira, Renata Cristina Mendes, Francieli S. Ruiz, and Marco Túlio de Mello. "Human sleep and immunity: The role of circadian patterns." Handbook of Clinical Neurology 206 (2025): 93-103.
6. Aristizábal B, González Á. Innate immune system. In: Anaya JM, Shoenfeld Y, Rojas-Villarraga A, et al., editors. Autoimmunity: From Bench to Bedside [Internet]. Bogota (Colombia): El Rosario University Press; 2013 Jul 18. Chapter 2. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459455/
7. NHS inform. Chickenpox [online 2024]. Available at https://www.nhsinform.scot/illnesses-and-conditions/infections-and-poisoning/chickenpox/. [Accessed June 2025]
8. NHS inform. Immune system [online]. Available at https://www.nhsinform.scot/illnesses-and-conditions/immune-system/. [Accessed June 2025]
9. Manchester University NHS Foundation Trust. Infection and immunity [online 2025]. Available at https://mft.nhs.uk/the-trust/other-departments/laboratory-medicine/immunology/clinical-conditions-covered-by-the-immunology-department/infection-and-immunity/. [Accessed June 2025]
10. NHS. Insomnia [online 2024]. Available at https://www.nhs.uk/conditions/insomnia/#:~:text=On%20average%3A,need%2012%20to%2017%20hours. [Accessed June 2025]
11. 11. NHS inform. Health benefits of eating well [online 2025]. Available at https://www.nhsinform.scot/healthy-living/food-and-nutrition/eating-well/health-benefits-of-eating-well/. [Accessed June 2025]
12. NHS. B vitamins and folic acid [online 2020]. Available at https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-b/. [Accessed June 2025]
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14. NHS. Vitamins and minerals: others [online 2020]. Available at https://www.nhs.uk/conditions/vitamins-and-minerals/others/. [Accessed June 2025]
15. NHS. Iron [online 2020]. Available at https://www.nhs.uk/conditions/vitamins-and-minerals/iron/. [Accessed June 2025]
16. NHS. Eating a balanced diet. Available at https://www.nhs.uk/live-well/eat-well/how-to-eat-a-balanced-diet/eating-a-balanced-diet/. [Accessed October 2025]
17. NHS. Benefits of exercise [online 2024]. Available at https://www.nhs.uk/live-well/exercise/exercise-health-benefits/. [Accessed June 2025]
18. Alotiby A. Immunology of Stress: A Review Article. J Clin Med. 2024 Oct 25;13(21):6394. doi: 10.3390/jcm13216394. PMID: 39518533; PMCID: PMC11546738.
19. NHS inform. How stress symptoms affect your body [online]. Available at https://www.nhsinform.scot/mind-to-mind/handling-stress/how-stress-symptoms-affect-your-body/#:~:text=stress%20releases%20cortisol%2C%20suppresses%20our,in%20a%20more%20irritable%20state. [Accessed June 2025]