The gut-immune axis: How probiotics help train your body‘s defences for winter

The gut-immune axis

The digestive tract is home to the most extensive immune network in the body, with an estimated 70% of our immune system residing within the gut.¹ This is because it is the largest point of contact between the external world and our internal body. Every single day, we‘re constantly introducing substances from the outside world directly into our bodies via the digestive tract — food particles, microorganisms like bacteria, fungi, and viruses, and other matter. Some of this may pose a risk to our health, while others may even be beneficial to us. Our immune system must be present and active in the gut where our inner world meets the outer world, so it can learn tot ell the difference between what is harmful and what is helpful, and then decide how to respond appropriately. This critical relationship between the gut and the immune system is what scientists call the ‘gut-immune axis’, and it‘s where much of the immune system‘s learning and decision-making takes place.

The immune system in the gut

The gut is home to several key players that work together to help educate the immune system, ensuring it can tell the difference between friend and foe, and respond appropriately:

1. The gut microbiota: the microorganisms living in the gut microenvironment
2. The gut lining: a thin layer of cells with a protective mucus layer overlaying it
3. Gut-associated lymphoid tissue (GALT): specialised immune tissue in the gut

Immune system training

The GALT is highly complex tissue that houses many different immune cells.² Some of these cells act like rangers, continuously sampling the contents of the gut and extracting small pieces (known as antigens) which they then present to more specialised immune cells for assessment. These specialised cells then evoke a highly specific response based on whether the antigen comes from a beneficial, harmful, or neutral source, triggering a cascade of communications with other immune cells to orchestrate an appropriate response.³

Over time, repeated exposure to specific antigens helps the immune system learn when to respond in a strong, targeted way (e.g. against threats like viruses) and when to lay low. The latter would be appropriate when we don’t want the immune system to mount an excessive response against dietary antigens from the food we eat, or against beneficial microorganisms such as those living in the gut, or the ones we consume as probiotics.

Why does a balanced immune response matter?

A balanced immune response is essential for good health. If the immune system is insufficiently trained, the body could become more susceptible to infections. On the other hand, it could become hyperresponsive and overreact to matter that is usually unproblematic, such as certain foods. In this case, it contributes to conditions like allergies. For these reasons, supporting the gut and the gut microbiota is paramount, so that the gut-immune axis can function optimally and help you stay well.

How does the gut microbiota train the immune system

The state and composition of your gut microbiota has a significant impact on immune function.⁴ Beyond simply residing in the gut, beneficial microorganisms play an active role in shaping and training the immune system. They do this in several ways:

• Interacting directly with immune cells in the GALT
• Producing substances that influence immune responses
• Helping to maintain the integrity of the gut lining, which acts as a defensive barrier between the gut‘s contents and the rest of the body

In this way, a diverse and balanced microbiota helps ensure the immune system remains well-calibrated: responsive enough to deal with genuine threats, but not so overactive that it causes harm.

What happens to your immune system when the gut microbiota is distrupted

When this balance is disrupted, with fewer beneficial microorganisms and more problematic ones, it is referred to as gut dysbiosis. Without the right microbial input, the immune system can lose some of this careful calibration, becoming either under- or over responsive. Dysbiosis can come about due to many factors, including:
• Antibiotic use — reducing the diversity of microbial species in the gut microbiota⁵
• Poor diet — especially insufficient fibre intake, as fibre fuels good gut bacteria⁶
• Frequent infections — this can also reduce microbiota diversity⁷ and can overburden the protective gut barrier, placing excessive strain on the gut-immune axis
• Stress and poor sleep quality — known to deplete healthy gut flora⁸ and reduce our capacity to fight illness⁹

For the immune system to respond appropriately to potential threats (e.g. a virus which may cause an infection), the gut microbiota, gut lining, and GALT all need to be functioning well. Fortunately there are accessible tools such as probiotics which can address each of these aspects of the gut-immune axis in a targeted way.

Can probiotics strengthen your immune system?

Yes, probiotics participate in ‘immune training‘ in the gut and help strengthen the immune system to fight off and prevent illness in several ways:

• Producing defence substances — some probiotic strains produce antimicrobial peptides which actively work against pathogens.¹⁰
• Strengthening the gut lining — certain strains help strengthen the mucus layer of the gut lining so infectious pathogens struggle to take hold in the first place.¹⁰
• Stimulating white blood cells — certain strains have been shown to increase the number and activity of white blood cells,^11,12 which are crucial immune cells that help fight off infections.
• Enhancing immune signalling — others have been shown to enhance the secretion of important signalling chemicals from immune cells,¹³ helping specialised immune cells to carry out their specific functions more effectively.
• Supporting microbial coordination — while probiotics don‘t remain in the gut indefinitely, their activity as they pass through helps beneficial bacteria already living there to communicate more efficiently with each other and with the immune cells beneath the gut lining.

Because they participate in the gut‘s immune training process, their benefits extend well beyond the gut itself. When we zoom back out of the gut and into the real world, we see that certain probiotics show real results for the people who take them.

Clinically trialled probiotics to support immunity

Learning how probiotics can help stimulate immune function is fascinating, and what is even more exciting is when we can see the impact they have for real people by observing the results of clinical studies. Specific probiotic strains can help reduce recurrence of infections, such as the common cold.

In two clinical trials investigating a combination of the strains Lactobacillus plantarum HEAL9 and Lactobacillus paracasei 8700:2, participants who took these probiotics daily over a 12-week winter period experienced 29% fewer colds compared to people who took a placebo,¹⁴ as well as a 38% reduction in the severity of their cold symptoms if they did get sick.¹⁵ Importantly, these benefits were seen when probiotics were taken consistently and preventatively to give the immune system time to learn and respond to them. The key is regular, ongoing use rather than waiting until illness strikes. These findings demonstrate how specific probiotic strains, when carefully selected, can make a meaningful real-world difference to immune health.

Summary

The gut-immune axis is a complex system with many moving parts, all of which need to be functioning well for the body to respond to infections in a healthy, robust way. Specific probiotics offer a specific, targeted strategy to support this system; strengthening the gut barrier, nourishing the gut microbiota, and supporting the specialised immune tissue within the gut itself. In doing so, they help train the broader immune system to better recognise and respond to pathogens, keeping the body resilient when it matters most.