The GI immune system starts at the mouth and ends in the anus. The system is designed to allow our bodies to receive nutrition from food and drinks, while recognizing beneficial organisms and defending from harmful organisms. The GI system is a tube which is actually considered to be on the outside of our body. Just like the skin (lined by durable keratinized epithelium) is the first defense against the outside, the epithelial lining of the GI system is the first line of defense against anything that goes in our mouths. Note: much of the material here is plagiarized from a report I wrote on the GI system.
We chew our food which gets mixed with saliva. Saliva is amazing stuff. Adults make about 1 liter a day. Saliva contains different types of mucus (another amazing substance); enzymes to break down food; peroxidases, lactoferrin, lysozymes, and IgA to protect against microorganisms. Many of these things are also found in breastmilk, which protects infants who are born without memory in their adaptive immune system. Tight junctions hold the epithelial cells together, and hard keratin protects surfaces which get more friction. Nerves take information to our brain to decide if our teeth are biting something too hard, how the food tastes, to decide if we need more saliva.
No only do we have to defend against food, but also stuff we breathe in. The tonsils use the power of the adaptive immune system to help out. The adenoids (pharyngeal tonsils) are in the back of the nasal passage and deal with air and mucus from the nose. The (palatine) tonsils are directly behind the mouth, and under the adenoids. The lingual tonsils sit in the back of the tongue. These structures are composed of lymph tissue, similar to lymph nodes.
A Node on Lymph Nodes
Lymph nodes are like military bases. When antigen presenting cells find a prisoner (an interesting protein), they can bring it to their local lymph node, which contains T cells and B cells. The T and B cells can interact with one another and make memory cells, and plasma cells which make antibodies. Lymph vessels carry extra fluid from between cells to lymph nodes for processing. It seems there is not a place in the body where you can escape from the T or B cells.
From the esophagus to the anus, the system is more or less a tube, with modifications in each area. The tube has 3 layers: the inner layer (mucosa); middle layer (submucosa) which contains connective tissue, nerves, blood and lymph vessels; and an outside layer of muscles which moves the food in a process called peristalsis.
The mucosa itself has 3 layers: epithelium, lamina propria, and a thin muscle layer. The epithelium has different cells for different jobs in each organ. The lamina propria contains connective tissue, white blood cells, and lymph tissue.
The esophagus brings food to the stomach. This is basically a tube lined with boring squamous cells like in the mouth. These cells don’t absorb, but they are protective and replaced often, so they can handle the rough food going down. People who have reflux can be exposed to stomach acid which changes the cells from squamous cells to stomach-lining cells, and this can eventually cause cancer. There are glands in the esophagus that secrete enzymes and mucus. The glands are tubes that go down through the layers of the esophagus.
The stomach is a widening of the tube system, and it has a lot of muscle mass so it can mix the food like a blender. The strong acids can kill many of the organisms that find their way inside. Mucus producing cells protect the lining of the stomach from being destroyed by the acids. Glands in the stomach produce enzymes and hormones. The hormones can tell the muscles when to contract to push down food, and also sends signals to other organs that it needs other enzymes/ hormones.
The small intestine is huge and has a massive amount of surface area, around th size of a football field. Its cells are absorptive, so this leaves the body very vulnerable to anything that enters here. So a really super defense is needed. The lining of the small intestine is replaced every 5 days. It’s villi are fingerlike projections that increase the surface area for absorbing food and water. And its glands (Crypts of Lieberkuhn) go down into the lamina propria. The crypts are lined by absorptive cells, enzyme and hormone producing cells, and “M” cells. Peyer’s patches are groups of lymph tissue nodules found at the end of the small intestine, which are covered by M cells. M cells eat (phagocytose) substances in the intestine, and then pass them off to antigen presenting cells in the nodes. Inside the lymph nodes B cells are produced, which go to the lamina propria, differentiate into plasma cells and produce IgA. After that some B cells go on further into the lymph system and magnify the immune reaction. Some of the IgA is attached to a chemical called “secretory component” or sIgA, which is then to the cell membrane of the surface intestinal cells. The rest of the IgA enters the liver, turns into sIgA, and is released into the bile. Most of the nutrients are absorbed into the blood and carried right into the liver for processing. The liver also recycles bile from the intestines. Bile is repackaged and goes back into the beginning part of the small intestine (via the common bile duct), along with IgA, and more enzymes/hormones from the pancreas.
The large intestine is similar to the small with its crypts and lots of absorptive cells, but it doesn’t have villi. The appendix is located near the terminal ileum, and it also has M cells and lymph nodules (interesting…). The colon absorbs water and gas and compacts the food into feces for elimination:)
Could talk more about these. We are supposed to have layers of good bacteria in our intestines. The good bacteria do a lot of digestion for us and stimulate production of IgA and provide short chain fatty acids. In addition, we evolved to host soil transmitted helminths, which are symbiotic and regulate our immune system.
- The wiki on Peyer’s patches says “Although important in the immune response, excessive growth of lymphoid tissue in Peyer’s patches is pathologic, as hypertrophy of Peyer’s patches has been closely associated with idiopathic intussusception.” Intussusception is why the original rotavirus vaccine was taken off the market. It is known is that injected vaccines activate different cytokine cascades than mucosal exposure. It’s possible that stimulating the immune system in an abnormal way is causing immune diseases like autism and eosinophilic esophagitis. I hope more research will be done on these topics.
- What is the real cause of appendicitis?
- Radiohead’s Jigsaw Falling into Place is about the GI system. “Before you run away from me Before you’re lost between the
notes epithelial cells The beat bile goes round and round… …Come on and let it out”