The Real Link Between The Gut & Immunity

There has been for some time a lot of chatter about ’70% of the immune system being in the gut” or whatever the quoted number is this week. These blanket statements are seldom ever true or accurate, and…no….that one isn’t either!

However, the immune system and the gut ARE closely linked and of course the microbiome is a key part of this. In this article I wanted to explore this relationship and give you a more accurate picture.

The Microbiome - The Immune System’s Helping Hand

The microbiome is of course the bacterial colony that lives within our gut. Throughout the digestive tract we have different areas with different types of microbes, in differing amounts, all with different jobs to do. It is involved in digestion, metabolism, nutrient synthesis, and of course it is one of the key regulators of physiology - more than we ever imagined possible. One key area that it does play a real helping hand in is immunity (Hooper et al, 2012).

The microbiome influences immunity in 2 ways. It educates and it regulates. Firstly it stimulates and oversees the maturation of key cell lines, helping them go from juvenile to fully formed. During this process it also is involved in the production of antibodies that help to defend against pathogens.

The microbiome also helps to train immune cells that stay local to the gut, tolerance. This is the term that is given to the immune system being able to identify food stuffs, vitamins, minerals, amino acids, phytochemicals etc and let them pass through and be absorbed, and at the same time recognise something that shouldn't come in and shut the door to it (Belkaid et al, 2014).

Part of this control and development is derived from short chain fatty acids. These are by products secreted by the microbiome when it ferments and breaks down complex carbohydrates. These substances have for example been shown to regulate the function of T and assisting in the promotion of an anti-inflammatory immune state (Furusawa & Yukihiro, 2013).

The Role of The ‘Barrier’

You may at times have heard me talking about ‘barrier function’. This is the name given to three main areas - the microbiome itself as a biofilm, the mucous layer, and the tight junction. Let’s look at each one in turn.

The biofilm - imagine so much of the gut’s internal surface covered in bacteria that it is almost like a film wrapped over it, or a force field. Well, a healthy microbiome will very much be like that. As such it can be a physical barrier that prevents pathogens from getting through. Add to this the fact that many of the beneficial strains of bacteria in the gut secrete substances such as bacteriocins that kill off problematic bacteria, the biofilm can be an incredibly effective first layer of defence.

The mucous layer - this is as the name suggests a layer of mucous on the gut wall that is what the microbiome actually sticks to. This sticky layer is another physical barrier that makes life extremely difficult for pathogens that struggle to penetrate it.

The tight junction - the gut wall is a single cell in thickness. The enterocytes (gut wall cells) are very tightly bound together by bands of protein. Imagine you had 2 matchboxes next to each other, and then wrapped 10 elastic bands around them to pull them as tightly together as feasibly possible without damaging them. Well, its the same principal. The tight junction is bands of proteins that pull the cells that make up the gut wall tightly together so that any pathogen that gets past the first 2 lines of defence, they will not be able to squeeze through the cells of the gut wall and enter the body.

Disruption of this barrier, often referred to as "leaky gut," has been linked to various autoimmune and inflammatory diseases, indicating its critical role in immune homeostasis (Mu, et al., 2017).

Dietary Influences on Gut Health and Immunity

Diet plays a significant role in shaping the gut microbiota composition and, consequently, the immune response. Diets rich in fiber, for example, support the growth of beneficial bacteria that produce SCFAs, known to enhance gut barrier function and reduce inflammation. Conversely, a diet high in saturated fats and processed foods can lead to dysbiosis, an imbalance in the gut microbiota associated with increased gut permeability and inflammation (Singh, Rasnik K., et al., 2017).

What about probiotics and prebiotics?

The administration of probiotics and prebiotics has been explored as a strategy to modulate the gut microbiota and enhance immune function. Probiotics, live microorganisms that confer health benefits, have been shown to improve gut barrier integrity, reduce inflammation, and enhance pathogen defence (Hill, Colin, et al., 2014). Prebiotics, non-digestible fibres that stimulate the growth of beneficial bacteria, also play a role in maintaining immune health by supporting the production of SCFAs and modulating the gut microbiota composition (Slavin, Joanne, 2013).

References

Belkaid, Y., & Hand, T. W. (2014). Role of the microbiota in immunity and inflammation. Cell, 157(1), 121-141.

Furusawa, Y., et al. (2013). Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature, 504(7480), 446-450.

Hill, C., et al. (2014). Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology, 11(8), 506-514.

Hooper, L. V., et al. (2012). Interactions between the microbiota and the immune system. Science, 336(6086), 1268-1273.

Mu, Q., et al. (2017). Leaky Gut As a Danger Signal for Autoimmune Diseases. Frontiers in Immunology, 8, 598.

Singh, R. K., et al. (2017). Influence of diet on the gut microbiome and implications for human health. Journal of Translational Medicine, 15(1), 73.

Slavin, J. (2013). Fiber and prebiotics: Mechanisms and health benefits. Nutrients, 5(4), 1417-1435.