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 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 a 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 are 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 like 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.