The digestive system is the only route of entry for food and other substances into the body. It is safeguarded by the immune system to keep the pathogens away.
However, the digestive system anatomy and physiology are in such a way that it acts as a physical and chemical barrier for pathogens and also facilitates the immune system to exert control over harmful organisms.
These critical systems work together efficiently to maintain the body’s health and homeostasis.
How the digestive and immune systems interact.
The two systems work together in combating the entry, growth, and multiplication of microbes and parasites in the body.
The innate immune system has different components, like physical, chemical, and physiological barriers, and the digestive system has all of these components.
Physical Barrier Protection
- The digestive tract (especially the gut lining) acts as a physical barrier to prevent harmful microbes from entering the body.
- This tract is made up of 4 layers of tissue, like the
- Mucosa
- Sub-mucosa
- Muscular and
- Connective tissue layer
- The mucosal layer exposed to food acts as a physical barrier for the entry of microbes into the bloodstream.
- In fact, when the microbe load is high, there would be shedding of mucus, which goes out of the gut, eliminating microbes through feces.
- A recent MIT study showed that mucus produces mucins (high molecular weight proteins).
- These mucins adhere and prevent the microbes from attaching to other tissues and cells, making them harmless.
Medical Cases:
- In case of severe intestinal infections like Bacillary Dysentery, amebic dysentery, the stools contain mucus, including the disease-causing pathogen, as in cholera.
- Here, due to excess microbial activity, the mucus gets damaged and sheds off. It also carries away pathogens.
- The body, as a compensation, also generates more mucus.
Chemical barrier
- The gut also secretes chemicals that can kill and neutralize the invading microbes and pathogens, like worms.
For this, it secretes
- Stomach acid
- Lysozyme enzyme
- Other Antimicrobial peptides.
Stomach acid
- The stomach secretes Hydrochloric acid as a part of gastric juice.
- This hydrochloric acid keeps the pH of the stomach at 1.5, which is highly acidic and can destroy most forms of living matter, including bacteria, worms, and any live animals entering the stomach.
- At this acidic pH, the proteins and enzymes of the pathogens get denatured, killing them after exposure.
- Thus, stomach acid is a powerful defence against invading pathogens and microbes.
Lysozyme enzyme
- Lysozyme is the enzyme found in gut secretions like
- Saliva,
- Gastric juice,
- Intestinal secretions by paneth cells, and
- The mucus tissue secretions.
- This lysozyme enzyme specifically destroys the peptidoglycan layer by hydrolysis of glycosidic bonds in the bacterial cell wall.
Physiological barrier
- This includes some physiological effects like
- Intestinal motility
- Secretions
Intestinal motility
- The digestive tract has motility, where the gut contracts and also shows peristalsis, segmentation, and secretion.
- Due to the peristalsis and movements, the pathogens are not allowed to adhere, settle, and are flushed out with other waste out of the body.
- Without these movements, there could be microbial overgrowth and constipation.
Secretions
- Further, the intestine also shows profuse secretion during infections,
- This secretion enhances the bulk and helps dilute the toxins released by pathogens and thereby removes them from the gut.
- Besides, these secretions also contain Secretory Immunoglobulin A (sIgA), which binds to pathogens and neutralizes them.
Gut-Associated Lymphoid Tissue (GALT)-Peyer’s patches:
- The small intestine contains lymphoid nodules called Peyer’s patches that are specialized immune tissues
- They are found in the distal part of the ileum of the small intestine and are meant to detect and respond to pathogens.
- These Peyer’s patches keep sampling the intestinal content and help recognise harmful and non-harmful antigens before launching an attack on a particular antigen.
- This way, they bring in homeostasis in the gut by helping the commensal bacteria survive and harmful bacteria be targeted by the antibodies and immune cells.
Cases:
- In the case of typhoid, Salmonella bacteria target these Peyer’s patches, leading to inflammation and necrosis of the ileum.
- Similarly, Inflammatory bowel disease (IBD) is thought to be due to dysfunction of Peyer’s patches.
Good Bacteria (Microbiome)
- The gut is home to many friendly bacteria, like the
- Lactobacillus species
- Bifidobacterium species
- Bacteroides species,s etc.
- These bacteria are helpful as they are involved in functions like breaking down complex food, synthesizing vitamins like K, B12 required for the body.
- Further, they support immune function by outcompeting harmful microbes and producing antimicrobials that prevent the growth of pathogenic bacteria.
Immune Cell activation and protection
- Lamina Propria is a layer of loose connective tissue below the mucus membrane of the small and large intestine.
- This acts as a key site of immune defense by hosting 70 to 80% of the body’s immune cells, like
- T cells
- B cells
- Plasma cells (which produce antibodies like IgA)
- Macrophages
- Dendritic cells
Mast cells
- These cells are activated in response to pathogenic microbes or their antigens and provide a line of immune defense.
Cases:
IBD (Inflammatory bowel disease) and autoimmune reaction to gluten are due to overactive lamina propria, which leads to gut inflammation, ulceration, pain, and diarrhea.
Nutrient Absorption
- The digestive system absorbs nutrients, including vitamins A, C, D, and zinc, that are essential for a strong immune response.
- These nutrients support the immune system and help in the defense of the body.
| Nutrient | Effects on disease. |
|---|---|
| Vitamin C | An antioxidant that stimulates formation of antibodies by immune cells. |
| Vitamin D | Regulated immune responses. |
| Vitamin A | Keeps the gut lining strong. |
| Vitamin E | Antioxidant and also supports T cells function |
Cases:
It is a scientific fact that a lack of proper nutrition is the cause of many life-threatening diseases, like tuberculosis.
References:
- Fighting bacteria with mucus
- Mucus and Mucins: The Underappreciated Host Defence System
- Applications of Lysozyme, an Innate Immune Defense Factor, as an Alternative Antibiotic
- The Paneth Cell: The Curator and Defender of the Immature Small Intestine
- Multi-faceted functions of secretory IgA at mucosal surfaces
- Role of intestinal epithelial cells in the host secretory response to infection by invasive bacteria.
- The Roles of Peyer’s Patches and Microfold Cells in the Gut Immune System
- The gut microbiota and mucosal T cells