According to the definition of the World Health Organization (WHO), probiotics are “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host”. They are found in our diet or in the form of dietary supplements.
They come to lend a helping hand to the gut microbiota, which is made up of all the micro-organisms already present in our digestive system.
Among other things, they are credited with benefits for the relief of digestive discomfort, side effects related to antibiotics, or even during weight loss.
Among these benefits, it is often said that probiotics could support the immune system. But what is it really?
Before getting to the heart of the matter, I want to remind you that probiotics are in no way medicines. However, they must be taken with care, and I recommend that you discuss them with a health professional before starting any course of treatment.
📚 Read also | Our tips for choosing the best probiotics
Gut microbiota and immunity: a now well‑established link
70 to 80% of immune cells are located in the gut
People often say that the intestines are the second brain. But did you know that they are also essential for the proper functioning of your immune system?
We now know that 70 to 80% of the body’s immune cells are located in the intestinal tract, particularly in the GALT (Gut‑Associated Lymphoid Tissue).
This lymphoid tissue houses various immune cells (lymphocytes, plasma cells, macrophages). It is also a major site of antibody production, enabling the body to defend itself against pathogens.
At the same time, the gut microbiota also plays an active role in immunity. Experimental studies show in particular that mice raised without microbiota display an immature intestinal immune system and a less effective response to pathogens.
Another study, reported by Inserm, confirms that the absence or immaturity of the microbiota is accompanied by increased vulnerability to infections, especially early in life.
Disrupted microbiota, impacted immune system
That’s not all. The microbiota also produces beneficial metabolites, called short-chain fatty acids (SCFAs), when breaking down certain dietary fibers. Scientific reviews indicate that these molecules help strengthen the intestinal barrier and influence the expression of genes involved in the immune response.
Conversely, when the microbiota is disrupted, the interaction with the immune system is altered. Studies have also shown an association between dysbiosis and immune disorders, with an increased susceptibility to inflammatory or autoimmune diseases.
Probiotics and immunity: what are the mechanisms of action?
They strengthen the intestinal barrier
The most well-documented effect of probiotics today is their involvement in strengthening the intestinal barrier.
Recent research, including a meta-analysis of 26 clinical trials, suggests that the probiotic strains Lactobacillus and Bifidobacterium may help improve the integrity of this barrier.
The study also showed that taking probiotics increases the abundance of these same Bifidobacterium and Lactobacillus strains within the intestinal tract, which helps limit the passage of endotoxins into the bloodstream.
I should still point out that the authors emphasize the need for additional studies to better clarify the effects of each strain on the body, depending on the dose and the individual.
They modulate the immune response
Probiotics do not act only on the gut flora: they may also interact directly with intestinal immune cells.
Specifically, when certain probiotic strains come into contact with the intestinal mucosa, they may be able to interact with local immune cells and influence the production of cytokines and IgA antibodies. This is, in any case, the conclusion of this scientific study.
Probiotics would therefore help to strengthen the immune defenses while limiting unnecessary inflammatory reactions.
New studies could be useful to better understand the mechanisms involved.
They act on the production of metabolites
I also found several studies showing that probiotics may stimulate the production of SCFAs, these metabolites that are beneficial for immunity. The mechanisms, however, have not yet been clarified today.
Which probiotic strains should be favored to support immunity?
Not all probiotics have the same effect. The research results showing benefits used specific strains at specific doses. So not just any probiotic will necessarily be useful to boost your natural defenses.
The most studied strains for immunity are Bifidobacterium and Lactobacillus.
In practice, I mainly recommend that you pay attention to:
- the presence of clearly identified strains
- a sufficient dosage, expressed in colony-forming units (minimum 1 billion CFU)
- an appropriate formulation (for example, gastro-resistant capsules)
- and a choice in line with your needs.
If you are not sure which strain to choose, talk to a healthcare professional.
According to the current scientific literature, there is indeed a link between probiotics and immune support, through the strengthening of the intestinal barrier and the modulation of the immune response.
Nevertheless, these effects are neither systematic nor universal. They depend on many factors and on each individual. I therefore encourage you to use probiotics as a complementary tool, alongside a healthy lifestyle and an appropriate diet.
Sources and scientific studies
- Oyovwi, M.O., Ajayi, A.F. (2025) A comprehensive review on immunological mechanisms and gut-brain pathways linking gut health and neurological disorders.
- Valentine Boutherin, Florence Piastra-Facon and Emma Risson (2019), The intestinal microbiota, a key modulator of immune physiology
- Zeng, JingjingHe, ZimengWang, GuoqingMa, YuxinZhang, Feng (2025), Molecular Mechanisms, Biological Functions, Diseases, and New Therapeutic Opportunities
- Fatima, S. N., Arif, F., Khalid, R., Khan, M., & Naseem, K. (2025). Immunomodulatory role of gut microbiota in autoimmune disorders and the advancement of gut microbiota based therapeutic strategies.
- Yanfei Zheng et al. (2023), Probiotics fortify intestinal barrier function: a systematic review and meta-analysis of randomized trials
- Mazziotta C, Tognon M, Martini F, Torreggiani E, Rotondo JC. (2023) Probiotics Mechanism of Action on Immune Cells and Beneficial Effects on Human Health.
- Liu Y, Wang J, Wu C. (2022) Modulation of Gut Microbiota and Immune System by Probiotics, Prebiotics, and Postbiotics.
