Scientists estimate that 70% of our immune system is housed in the gut. This is a combination of our own immune cells, as well as the immune benefits provided by our beneficial gut bacteria. It is clear therefore that ensuring digestion and gut health are in good working order can have a positive effect on our immune health.
Here we take a closer look at how our immune system works and the role that our gut bacteria, and probiotics, play in supporting it.
So, how does our immune system work?
This incredibly sophisticated system is constantly surveilling and monitoring our bodies for intruders, pathogens such as bacteria, viruses, parasites and fungi. Most of the time, its role is to do nothing, to recognise that the food just eaten or the air breathed is not harmful to our health and can be left alone. However, when we suffer an injury, or develop an infection, the immune system kicks in with a two-pronged approach.
The first step is our innate immune response, a non-specific reaction that is designed to promote healing and repair, replace any damaged cells and contain and remove any foreign bodies to prevent further harm. Physical symptoms caused by the innate immune system in response to an infection include inflammation, fever, fatigue and lack of appetite, all of which play a role in the healing process.
However, if the innate immune response is unable to contain the infection, our adaptive immune response kicks in. This recruits white blood cells – T-lymphocytes and B-lymphocytes – and antibodies. Unlike the innate immune response which reacts in the same way regardless of the cause, the adaptive immune response is specific to the virus or bacteria and has a memory (antibody) that can ensure a swifter reaction if that pathogen should cause an infection again at some point in the future.
How does our gut play a role in immunity?
There are four ways that microbes or pathogens can enter the body; through a cut in the skin, through the respiratory tract, the uro-genital tract or the digestive tract. When you consider that the vast majority of ‘foreign bodies’ are encountered in the food and drink we consume each day, it makes sense that the digestive tract is the primary location of our immune system.
The first defence against pathogens in our food is the highly acidic environment of the stomach, thanks to secretions of hydrochloric acid each time we eat which kill many bacteria before they reach the small intestine.
Next, is the physical barrier of the gut lining. The cells of the digestive tract are held together by tight junctions to ensure that, when working optimally, nothing can pass from within the gut to the blood. Unfortunately, some people can suffer from ‘leaky gut syndrome’, a condition in which the tight junctions have loosened, causing gaps between the cells of the gut lining that allow small particles of undigested food to cross the barrier and enter the bloodstream. When this happen, an immune response may be launched as the body recognises a foreign, ‘non-self’ substance within the blood. This is classed as a food sensitivity and causes symptoms such as bloating and fatigue.
In addition to the physical barrier, the cells of the digestive tract are covered in a mucus layer that traps pathogens and prevents them from entering the body. Our gut microbiota, the collection of trillions of beneficial bacteria living in harmony with the cells of our body, form part of the mucus layer in the intestines and are also understood to play a key role in both the development and activation of the immune system. Studies with germ-free mice have shown that an appropriate immune response may be impaired by a lack of beneficial bacteria within the digestive tract.
A physical way in which beneficial gut bacteria play a role is by limiting receptor sites available for pathogenic bacteria to latch onto within the gut and thereby crowding them out. They also compete with pathogenic counterparts for nutrients.
However, the beneficial bacteria have also been shown to play a far more sophisticated role , interacting with key cells of the immune system and, according to studies, exerting either a pro- or anti-inflammatory effect depending on the need of the host.
Ways in which probiotics can support immune health
If our beneficial gut microbiota play an important role in our immune health, then it stands to reason that supplementing with probiotics can also have a positive effect on the immune system. While probiotics have only a transitory period in which to exert their influence – typically lasting 72 hours within the digestive tract – their immune-supporting effects can include:
- stimulating the production of key inflammatory molecules that in turn either trigger or suppress inflammation as needed
- activating T-lymphocyte white blood cells, also known as T-cells, and guiding their response
- strengthening the mucosal layer of the digestive tract
- suppressing the growth of pathogenic, or disease-causing, bacteria
- strengthening the integrity of the gut barrier
- producing anti-bacterial substances to eliminate pathogenic bacteria
In addition to affecting immunity via the gut, we now understand that probiotics taken orally also have the ability to exert an effect on infections in other parts of the body, for example helping to combat UTIs in the uro-genital tract.
Can probiotics be taken long term to support immunity?
The good news is that while probiotics are understood to influence immunity, activating immune cells and triggering the production of inflammatory chemicals, this activity has been shown to have no effect on the ‘normal’ balance of the gut. As they are transitory, any immune response that they exert does not negatively affect the health of the gut, and they are recognised as being safe to take in the long term.
Cristofori, F. et al. (2021) ‘Anti-inflammatory and immunomodulatory effects of probiotics in gut inflammation: a door to the body’, Frontiers in immunology 12, doi:10.3389/fimmu.2021.578386
Sharifi-Rad, J. et al. (2020) ‘Probiotics: versatile bioactive components in promoting human health’, Medicina (Kaunas, Lithuania), 56(9), doi:10.3390/medicina56090433
Yan, F. and Polk D.B., (2011) ‘Probiotics and immune health’, Current opinion in gastroenterology, 27(6), doi:10.1097/MOG.0b013e32834baa4d
Round, J. and Sarkis K. (2009) ‘The gut microbiota shapes intestinal immune responses during health and disease’, Nature reviews. Immunology, 9(5), pp.313-23, doi:10.1038/nri2515