60 second summary
- Our gut microbes reside inside the opening of our intestines and a top layer of mucus
- They are separated from our gut cells by a protective bottom layer of mucus
- Immune cells can travel through the layers sampling for disease causing bacteria
A healthy gut is abundant in microbes called probiotics.
Probiotics- live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host
These are divided into 3 distinct categories: Bacteroidetes, Firmicutes and Proteobacteria which reside in the opening of your gut called the intestinal lumen and its top layer of mucus. Your gut has a sophisticated structure in which it divides the bacteria from your own cells, whilst still allowing the movement of immune molecules for defense.
What are the layers in the gut?
The top layer is the lumen which comes in direct contact with the food you eat.
Next, there is a double layer of mucus. The top layer, in contact with the lumen, is the habitat of many of your microbes: it is spacious with the ability to expand fourfold in volume.
The lower layer of mucus acts as a filter preventing bacteria from entering.
Blow this is the single cell barrier of your own cells called the epithelium.
The science: Overall, there are 4 types of epithelial cells:
1. Enterocytes- which absorb molecules from the intestinal lumen
2. Paneth cells- important in defense by releasing anti-microbial peptides when encountering pathogens
3. Goblet cells- secrete mucus
4. Entero-endocrine cells- produce hormones such as insulin.
The bottom layer of the gut lining is called the lamina propria which is where most of the immunity related cells reside.
The science– one important immunity related cell in the lamina propria is the dendritic cell. The function of these is to take samples from the mucus. If a harmful microbe is encountered the dendritic cell migrates to lymph nodes, sites densely populated with disease fighting T cells, where it activates an immune response.
How is our gut important in immunity?
The epithelial cells lining the gut form the largest interface between our external and internal environment.  Our gut microbes are responsible for maintaining this barrier’s integrity by communicating with our immune system.
The science: main mechanisms by which the barrier’s integrity Is modulated are:
1.Increasing activity of immune cells such as macrophages which locate and ‘eat’ foreign cells
2. Secretion of antibodies which neutralize pathogens
3.Secretion of mucus- a sufficient quantity of mucus prevents harmful bacteria from getting in contact with our own cells
4.Competition with pathogenic bacteria for resources such as nutrients and space
Overall our immune system is tightly linked to our microbes and adequate communication between the two is essential in maintaining homeostasis, in other words balance.
 Springer Link. (2018). Probiotics and the Gut Immune System: Indirect Regulation. Available at: https://link.springer.com/article/10.1007%2Fs12602-017-9322-6 [Accessed 17 Jul. 2019].
 PMC. (2013). Role of mucus layers in gut infection and inflammation. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3716454/ [Accessed 17 Jul. 2019].
 Mucosal Immunology. (2015). 4th ed. Academic Press, p.16.
 Study.com. (n.d.). Macrophages: Definition, Function & Types. Available at: https://study.com/academy/lesson/macrophages-definition-function-types.html [Accessed 17 Jul. 2019].