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Basic Immunology of the Digestive Tract that Everyone Should Know

By Edited Aug 28, 2016 1 1

Abdominal Cavity
What goes on in the gut is extremely important for our health. Recent research suggests the digestive system may hold the keys to controlling obesity and even modulating our mood. To help you get a handle on this new information, here is some background on the immunology in the gut.


Basic Immunology of The Intestines

The intestinal tract forms a sheath that encompasses the intestinal interior, which is called the lumen. This is the space within the digestive system where the waste is produced. For most people, the only contact that they've had with the intestinal sheath occurs when they consume a natural casing hot dog or a sausage packed in pig intestine. The parts of the intestines utilized to produce sausage are the small intestines, and they can be up to 32 feet long in humans. The small intestine links to the stomach at one end, and after that, connects to the cecum, which in humans is the starting point of the large intestine, and is the location of the appendix. The large intestine is considerably shorter than the small intestine, and is the most bacteria rich area of the intestines. It is here where the majority of the fluid is absorbed from the fecal material.

In a living intestine, the inside wall is covered with a mucus layer, which is thicker within the large intestine than within the small intestine. Just below the mucus layer is the external most protective layer of cells referred to as the epithelial cell layer. This layer contains epithelial cells and goblet cells that create mucus. Under the epithelial layer is space located between the epithelial cells and a ring of muscles (muscularis mucosae). This area is known as the lamina propria, and it contains many immune cells, small blood vessels referred to as capillaries, and glands that secrete mucus. The epithelial layer and the lamina propria are collectively known as the mucosa.

Colon Wall Layers
The mucosa in the colon is different from that found in the small intestine. In both the large intestine as well as the small intestine, the epithelial layer is molded into many compartments called crypts. Areas amongst the crypts that extend above the surface are called villi. The small intestine has both villi and crypts, the large intestine just has crypts. Microbial entry into the crypts is not allowed and Paneth cells located at the bottoms of the crypts safeguard them with the release of anti-microbial peptides, which kill microbes[1].

Also inside the mucosa, spread around, are lymphoid structures. Lymphoid structures are small, round, compacted areas full of immune cells. In the small intestine, they're referred to as Peyer's patches, and inside the large bowel, they're named colon patches. Although many immune cells dwell within the mucosa of the intestines, they are not completely isolated from the rest of the body. Transportation of immune cells happens by means of the blood circulatory system as well as via a component of the circulatory system called the lymphatic system.

The lymphatic system features vessels extending throughout the body and frequently lying parallel to the vessels. This system serves as an immune net with hubs known as lymph nodes. Lymph nodes are lymphoid structures containing numerous immune cells; a lot like the Peyer's patches and colon patches. The most critical and locally positioned lymph nodes of the intestines are named the mesenteric lymph nodes.

Underneath the mucosa are two further layers: the submucosa and the muscle layer. The submucosa consists of loose connective tissue and numerous blood vessels. The muscle layer contains the muscle tissues that manage the contractions of the intestines, which transport and compact stool. 

Immune Cells of the Digestive Tract

The key cells of the immune system located inside the intestine are macrophages, neutrophils, dendritic cells, mast cells, T and B cells. Each type has its own abilities, and immunologists classify them into two groups: innate immune system and adaptive immune system. But, they can be generally classified into battle, scouting and intelligence functions, which is for a beginner easier to understand. 

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Innate Immune System: Battle

Macrophage
Macrophages, mast cells and neutrophils function as the foot soldiers of the immune system, and they are part of what is called the innate immune system. These kinds are the most able to deal with bacterial intruders, and all are capable of phagocytosis; the engulfment and break down of microbes. Macrophages are the most well-known for their potential to ingest debris. One more skill, where macrophages and also neutrophils excel, is the respiratory burst. This is the discharge of dangerous oxygen radicals that are able to destroy intruders. Neutrophils and mast cells possess an additional weapon in the form of granules. Granules are little vesicles of harmful substances that can be released by them. Neutrophil granules contain mainly anti-microbial substances, while mast cell granules contain substances that trigger neighboring cells and lead to the signs of inflammation (redness, swelling and heat). Histamine, one of several primary factors that cause allergic responses, is released by mast cells. 

Innate Immune System: Scouting

Dendritic cells, as well as some macrophages, operate like the spies of the immune system. Dendritic cells, like the other battle forms, have the capability to phagocytize bacteria and other particles in the environment. In addition to enabling the dendritic cell to get rid of bacteria, this attribute also permits the cell to collect information about the invading pathogen. Employing receptors, dendritic cells can identify patterns linked with favorable and bad bacteria as well as viruses and fungi, which permits them to determine the full danger of an invader. However, what makes them actually great scouts is the fact that they also possess the capability to dismantle the engulfed microorganisms and deliver that info to the cells, which are specialists at intelligence. 

Adaptive Immune System: Intelligence

Despite the fact that the dendritic cells may dismantle bacteria and collect new evidence, they are not the cells that file away information about the bad guys. The cells in charge of the real intelligence functions are the T and B cells. These cells, unlike the other cell types, possess the ability to recall invaders from the past (like older people keeping in mind hazards which the younger generation isn't aware of). After they notice signs of recognizable pathogens, they're able to notify and manage all the other immune types. This process is what immunologists call an adaptive immune response.

Their memory is not formed utilizing little minds, but instead depends on complicated receptors known as the T cell receptor (TCR) and B cell receptor (BCR). TCRs and BCRs can easily identify small protein fragments. The memory is created when there is recognition of a pathogen-related molecule by the TCRs or BCRs along with other stimulatory signals from the environment verifying the danger. As soon as these triggers are present, T and B cells divide and become long-lived memory cells.

T cells

T cells interact primarily with dendritic cells, receiving info from them about local infections. The dendritic cells present pieces of the invader and other info in the form of cytokines (secreted proteins). The cytokines tell the it how terrible the danger is and, depending on the circumstance, the T cell will regulate its functionality and the alerts, which it delivers out.

T cells are separated into two primary types based on their attributes and specific protein markers found on the cell surface. T helper cells assist and trigger various other kinds and cytotoxic T cells are made to destroy infected cells. Cytotoxic T cells are a bit of battle and intelligence combined into one.

The first type are quite crucial since they can set up immune responses in the near future. Soon after being offered details about a local intruder by dendritic cells, they can morph into several types called T helper type 1 (Th1), T helper type 2 (Th2), T helper type 17 (Th17) or regulatory T cells (Treg). Th1 are designed for fighting internal microbes as well as viral infections, Th2 are produced mainly as consequence of parasite infection and also allergy, Th17 are generally formed when there are extracellular microbial infections, and regulatory T cells (Treg) usually are developed any time something is deemed as safe, and at the end of an inflammatory response. During safe situations, which are the majority of the time inside the intestinal tract, Treg are formed. It is these Treg that make sure that you do not have an immune response to the meals and other particles, which you consume[2].

B cells

These are not dependent on dendritic cells for the compiling of information. They're, for the most part, reliant on T helpers for their info. B cells, utilizing their BCR, locate soluble molecules floating about within the environment. Even so, they are not going to view the particle as a hazard until a T helper gives them an alert that it is linked with an attacker. The information is then transmitted via cytokines and receptor triggering. As soon as danger is communicated, the they produce antibodies and become long-lived memory B cells. Antibodies are extremely crucial weaponry against pathogenic agents. They're able to cover microbes, neutralizing their function, and, additionally, they inform other immune elements about danger. 

How Are Some Bacteria Beneficial?

When the gut is functioning properly, the majority of the luminal bacteria are harmless and actually advantageous at several levels. First, they have the capability to restrict the numbers of harmful bacteria inside the intestinal tract. This is performed by means of decreasing the digestive tract pH, the production of anti-microbial substances, and by out-competing bad bacteria for resources. Second, they have beneficial interactions with epithelial cells. It has been noted that whenever epithelial cells come into contact with microbes, they boost their function by improving the production of anti-microbial substances, mucus, and barrier proteins. This restricts the entry of dangerous bacteria into the layers beneath. The last method in which some bacteria are useful is by stimulating anti-inflammatory responses. By interacting with epithelial cells and dendritic cells, they're able to indirectly tell B and T cells to subdue harmful immune activation. This is why there is now so much research concerning good bacteria and probiotics. However, keep in mind that all bacteria once they enter your body have the potential to be harmful. It's only under controlled conditions of health that they can be beneficial[3].

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Immunology Associated with Two Kinds of Intestinal Disease

To get a solid idea of exactly how mucosal immune mechanisms perform during intestinal disorders, we'll quickly look at two types of diarrheal diseases: acute diarrhea and inflammatory bowel disease. Keep in mind that the first disorder can be easily cured without treatment, while the latter malady is unremitting and, not to mention, has no permanent cures.

Acute diarrhea

Acute diarrhea is usually attributable to the swallowing of bacterially contaminated food or water. The cause of the symptoms isn't usually a principal result from the body becoming overwhelmed by infection, but is typically a result of the intensive immune response that the microbes and their products bring about. During Salmonella infection, the epithelial cells and nearby battle types (macrophages, neutrophils and mast cells) all recognize and respond to the Salmonella. As a result, the intestinal wall structure turns into a battleground with degranulation, respiratory bursts, phagocytosis and cytokine secretion. Regrettably, the epithelial surface becomes a casualty of war in this predicament, and also the water absorbing aspect of the digestive tract is disturbed leading to diarrhea. Losing barrier integrity also makes it possible for the leakage of more microbes into the digestive tract wall and beyond. This will cause a systemic inflammation causing fever and weakness, and is also the explanation why food poisoning makes one really feel so terrible. Regulation is returned as soon as the Salmonella are eliminated, the immune cells settle down, and the epithelium is allowed to mend[4]

Inflammatory bowel disease

Inflammatory bowel disease (IBD) causes chronic inflammation within the digestive system. It is unknown precisely how this illness begins, but it is evident that it happens because of both genetic and environmental aspects. People suffering from IBD have massive numbers of immune cells in the inflamed regions of the gut. Impressive populations of neutrophils, macrophages, T and B types are found in particular. It is suspected that T and Bs are delivering instruction to the neutrophils and macrophages that consistently calls them into activity. The T helper response is frequently skewed away from Treg and is Th17. It is theorized that finding strategies to improve Treg would bring the system back into equilibrium allowing the inflammation heal.

As you can see, the mucosal immune system within the digestive system is a world of its own. Current research suggests that far more diseases than we realize stem for problems within the gut. It is not surprising that mucosal immunology is becoming an extremely popular field of research.

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Comments

Aug 14, 2014 9:14am
Yindee
This is ine of the best explanations of the gut and how it contributes to immunity. Well rated and I will refer to it often!
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Bibliography

  1. Kamada N. et al. "Role of the gut microbiota in immunity and inflammatory disease.." Nature Reviews Immunology. 13 (2013): 321-335.
  2. Lee, Y. et al. "Induction and molecular signature of pathogenic T(H)17 cells. ." Nature Immunology. 13 (2012): 991-999.
  3. Gill, N. et al. "The gut microbiota: challenging immunology." Nature Reviews Immunology. 11 (2011): 636-637.
  4. Masopust, D. et al. "Mucosal immunology: Infection induces friendly fire.." Nature. (2012): 41-43.

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