How Does Your Gut Affect Your Immune System?

How are the gut and immune system connected?

Your gut is home to a major part of your immune system, with around 70–80% of immune cells found in the gut. That makes sense: your gut is one of the main places your body meets the outside world, through food, drink and microbes, so it needs strong defences. Your gut bacteria and immune system are in constant contact, and that relationship helps train immune responses, maintain tolerance to harmless microbes and protect against real threats.

How does your gut microbiome support immunity?

Your gut microbiome supports your immune system in several connected ways:

• Training the immune system: Gut bacteria help your immune system learn the difference between harmless microbes and real threats, so it can respond appropriately.
• Producing helpful compounds: When gut bacteria break down fibre, they produce short-chain fatty acids and other compounds that help regulate immune activity and calm unnecessary inflammation.
• Protecting the gut barrier: A healthy microbiome helps maintain the gut lining, mucus layer and immune defences that keep harmful substances from crossing into the bloodstream
• Crowding out harmful microbes: A diverse, balanced microbiome makes it harder for harmful microbes to take hold by competing for space and nutrients.

Together, these help your immune system stay balanced, responsive to real threats without overreacting.

Can an unhealthy gut weaken your immune system?

An imbalanced gut may affect how well the immune system is regulated. When the gut microbiome loses diversity or balance, known as dysbiosis, it can disrupt gut barrier function, reduce colonisation resistance, and contribute to low-grade inflammation. In some cases, this has been linked with altered immune responses and greater susceptibility to infection. The gut is not the only thing that affects immunity, but a healthy gut microbiome is increasingly seen as an important part of a well-functioning immune system.

→ What Is Gut Dysbiosis?

What are the signs your immunity might be gut-related?

There’s no certain way to tell from symptoms alone, but some possible clues that your gut may be worth considering include:

• Frequent minor illnesses alongside digestive symptoms
• Ongoing tiredness or low energy
• Slow recovery from everyday bugs
• Digestive issues that appear during stressful or run-down periods

These are soft indicators rather than proof. Frequent illness, fatigue or slow recovery can have many causes, including sleep, stress, nutrition, deficiencies and underlying health conditions. But if several signs appear alongside digestive symptoms, your gut may be one part of the wider picture.

How can you support your immune system through your gut?

Supporting your gut may help support your immune system, and the steps are the same ones that build a healthy gut:

• Eat more fibre and variety to feed a diverse range of good bacteria
• Add fermented foods like yoghurt, kefir, and sauerkraut
• Limit sugar and ultra-processed food, which can drive inflammation
• Prioritise sleep, which supports both gut balance and immune function
• Manage stress, since ongoing stress affects both your gut and your immune response

These changes work gradually, so consistency matters more than quick fixes.

How can a gut microbiome test help?

A gut microbiome test can show you the balance of bacteria in your own gut, which is useful because everyone's gut is different. It won't diagnose an immune condition or measure how strong your immune system is, but it can give you a clearer picture of your gut as a starting point, so any changes you make are based on your own body rather than general advice.

→ Explore the Gut Microbiome Test
→ How to Improve Your Gut Health: The Complete Guide
→ What Is the Gut Microbiome?
→ What Causes an Unhealthy Gut?
→ Best Foods for Gut Health: What to Eat (and Why)