When you think about it, humans are just long donuts. There’s a long and winding canal that goes from our mouths to our anuses called the alimentary canal or sometimes the gastrointestinal tract. This tube starts and ends outside of your body, and it’s how you process and extract nutrients and materials from the food you eat. But it’s also a spot for potential pathogens to sneak in. Our digestive system interfaces with all kinds of nasty germs, and as a result, it’s armed with multiple immune system resources to protect our insides from the outside.
So today, we’re taking a tour of the alimentary canal with a focus on immunity. Digestion itself is a long process and involves everything from mechanically grinding down food in our mouths to treating food with enzymes so that we can better absorb nutrients.
Therefore, there isn’t just one immune system checkpoint — there are safeguards along with the entire system. Like the saliva in our mouths is really good at starting carbohydrate digestion, but molecular components in saliva can also act as the first layer of defense against pathogens. For instance, an enzyme in our saliva called lysozyme can slice open bacteria cells, and there are antimicrobial peptides like histatins that can inhibit the growth of certain fungi.
And some of those salivary glands have cells that make an antibody called immunoglobulin A which provides protection to the cells coating the mouth. But as we get closer to the organs that absorb nutrients into our bloodstream, we need more sophisticated safeguards in place. The first thing your food sees before being swallowed is the tonsils — bits of lymphatic tissue that provide a little more protection. While we might not care about them unless they get inflamed and we need them removed, they create an immune checkpoint between the mouth and the throat. After we swallow this chewed-up chunk of food, it goes down the esophagus — just a tube that transports that bolus of food from the mouth to the stomach.
And the stomach itself is a major contributor to chemical digestion, allowing us to create enzymes like pepsin to break down peptide bonds in proteins. But it also keeps us safe from ingested pathogens. The stomach is coated in glands called oxyntic glands which include cells called parietal cells that secrete gastric juice. Gastric juice is highly acidic, with a pH anywhere from 1 to 3.0.
It’s made up mostly of hydrochloric acid, which you might’ve experimented with in chemistry class. This creates a hostile environment that many microorganisms wouldn’t be able to survive in. As we continue down the canal, we see a bunch of defenses once we get into the small intestine.
It is, after all, where a lot of nutrient absorption happens during digestion. And if we’re absorbing anything into the bloodstream or lymphatic system, we need to make sure it’s not going to hurt us.
So it’s coated in a single layer of cells called the intestinal epithelium which creates a barrier against microbes. Some of these epithelial cells secrete mucins that keep microbes out of the more sensitive epithelium — kind of like a sticky fly trap. Others act like alarm systems that release chemical messengers when they detect a pathogen and recruit white blood cells to the scene. Then there are a bunch of little lymphatic tissue buds scattered around the intestine that we call, appropriately, gut-associated lymphoid tissue, or GALT. But we also have trillions of good microbes in our intestines which you might’ve heard of referred to as your gut microbiome.
It’s not the only colony of non-human microbes you have on your body either.
You have different microbiomes on your tongue and mouth, on your skin or vagina. And among the many digestive jobs, the gut microbiome has, it helps us defend against pathogens by competing for space and nutrients and also secreting substances that are toxic to those pathogens. Plus, they can trigger our innate immune response, a generalized, first pass of immune responses, when they detect potential infection and to help kick start our immune systems when we’re born. These gut microbes are present all throughout your gut and pop up again when we move into the next portion of our GI tract, a chamber called the cecum, which features a little dangly organ, the appendix.
In recent years, researchers have gotten rid of the idea that the appendix is a useless remnant of evolution and started to understand its role in immunity. The inner layers of the appendix are home to all kinds of immune cells like B and T cells, along with natural killer cells.
Plus, it houses a reservoir of those good bacteria that can replenish your normal gut microbiome in cases where the colon needs more like after diarrhea. Our large intestines, or colons, have similar immune defenses as our small intestine — more lymph tissue and a slightly different makeup of gut microbes. And those differences are due to how the large intestine doesn’t absorb nutrients into the bloodstream as the small intestine does.
By the end of the alimentary canal, your gut and immune system will hopefully have kept you safe from all kinds of potential ingested pathogens. Thanks for watching this episode of Seeker Human. If you want to learn more about the history of the immune system, check out this video we made a few months ago. Otherwise, make sure you’re subscribed so you don’t miss any future uploads from us.
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