Your immune system only attacks things it determines shouldn’t be there. By the time you’re around 2yo or so, the composition of your gut microbiome is pretty much established and your immune system recognizes the microbes there as “good guys”.
This is at least the current thinking and why it’s so incredibly difficult to change the composition of your gut microbiome. Something like a fecal transplant can change it, but it tends to drift back to where it was before the transplant. Basically your diet and environment in your first couple years of life or so will determine things like your chances of obesity for the rest of your life.
Your gut is a tube. Blood is outside* the tube, poop is inside the tube. The tube itself is a complex mechanism to move water and nutrients from the poop into your blood without ever mixing the two. If the two mix, your white blood cells will kill the gut bacteria, but probably not fast enough to stop you from dying from blood loss, or sepsis.
In short, do NOT get poop in your blood. Not even once. Not even to be cool at a party.
*mostly in its own tubes. You are, in fact, a series of tubes.
For what is worth, there are things (specifically viruses) inside your body that do kill bacteria in your gut and is thought to be part of the mechanisms through which their number is kept in check, but it’s not white blood cells on account of it not being your blood in there.
As others have said, white blood cells aren’t in your gut, as they’re too big to get through the cellular spaces making up your intestines.
Though immune antibodies do protect your intestinal lining, particularly IgA antibodies, IIRC. They help bind up pathogens, preventing them from working their way through the mucous and into your intestinal lining.
People are giving you really poor answers. The question you’re asking is why doesn’t our immune system attack commensal microbes, and it’s not just because they can’t reach the bacteria.
The antigens on commensal bacteria are capable of producing immunologic tolerance not unlike our own cells. This is because our immune cells are constantly being exposed to the microbes’ antigens without exposure to an adjuvant necessary for co-stimulation of that immune response; this repeated exposure (and lack of co-stimulation) results in our immune system saying, “hey, these microbes aren’t doing any harm, let’s flag these antigens so next time we see them we know not to bother reacting.”
Our gut also has immune cells called regulatory T lymphocytes (or Treg cells) that dull the immune response in this area. These Tregs secrete a protein called interleukin-10 (IL-10) which tells our white blood cells to stop presenting antigens. This is why a lot of vaccines have to be administered parenterally.
Big picture is to differentiate what’s part of the body and what’s not.
Your digestive tract is essentially a tube stretching between your mouth and anus. For all intents and purposes, whatever happens in that tube is “outside” your body.
Your immune system wouldn’t engage gut bacteria in the same way it doesn’t engage any of the microbiome living on your skin.
If you have Netflix, watch Cells At Work. They have a really great couple of episodes about this very topic. Its very simply explained and super cute.😀
The gut is one of the most magical places in the human body. Normally, you’re right, the immune system’s job is to destroy anything inside the body that is not recognized as “self.” But there’s bacteria everywhere from your mouth to your anus. in some ways your gut is just a long tube that is technically “outside” the body. But we have to do some magic to make non-sterile food into something we can bring into the body. So the GI tract serves as this special zone where the immune system has to tolerate a lot of bacteria and other “not-self” stuff like food without going crazy and attacking as food gets broken down into things we can absorb. There are many complex mechanisms in place to keep the balance. It’s not so simple as not having an immune system in your GI tract as some comments here seem to imply. Side note, when this mechanism doesn’t work well and your immune system does go crazy when it sees a bunch of “not-self” stuff in the GI tract, that causes inflammatory bowel disease. It’s like the special gut system brakes are taken off leading to inflammation.
How do white blood cells not kill all of the bacteria on your skin?
You don’t constantly have blood on your skin, so you also don’t constantly have blood in your bowels.
Bacteria in your gut are only a problem when they start going where they aren’t supposed to.
Fun fact: Staphylococcus aureus, one of the causes of Staph infections and MRSA (methicillin-resistant staphylococcus aureus) is a normal part of the bacteria on your skin and in your upper respiratory tract (sinuses.) It’s only when s. aureus enters somewhere it isn’t supposed to be that causes a nasty infection.
One is that white blood cells don’t live in your gut.
The other is that the bacteria in your gut help train your immune system to identify friend and foe. So the “good bacteria” in your gut are not eliminated by the immune system that does exist there.
Comments
White blood cells are found in blood. If you have blood in your gut something is seriously wrong with you.
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Because if you have blood in your gut, you have internal bleeding and should see doctor fast.
Your immune system only attacks things it determines shouldn’t be there. By the time you’re around 2yo or so, the composition of your gut microbiome is pretty much established and your immune system recognizes the microbes there as “good guys”.
This is at least the current thinking and why it’s so incredibly difficult to change the composition of your gut microbiome. Something like a fecal transplant can change it, but it tends to drift back to where it was before the transplant. Basically your diet and environment in your first couple years of life or so will determine things like your chances of obesity for the rest of your life.
The key word here is “blood”.
Your gut is a tube. Blood is outside* the tube, poop is inside the tube. The tube itself is a complex mechanism to move water and nutrients from the poop into your blood without ever mixing the two. If the two mix, your white blood cells will kill the gut bacteria, but probably not fast enough to stop you from dying from blood loss, or sepsis.
In short, do NOT get poop in your blood. Not even once. Not even to be cool at a party.
*mostly in its own tubes. You are, in fact, a series of tubes.
For what is worth, there are things (specifically viruses) inside your body that do kill bacteria in your gut and is thought to be part of the mechanisms through which their number is kept in check, but it’s not white blood cells on account of it not being your blood in there.
As others have said, white blood cells aren’t in your gut, as they’re too big to get through the cellular spaces making up your intestines.
Though immune antibodies do protect your intestinal lining, particularly IgA antibodies, IIRC. They help bind up pathogens, preventing them from working their way through the mucous and into your intestinal lining.
Maybe you meant to say stomach acid instead of WBC?
People are giving you really poor answers. The question you’re asking is why doesn’t our immune system attack commensal microbes, and it’s not just because they can’t reach the bacteria.
The antigens on commensal bacteria are capable of producing immunologic tolerance not unlike our own cells. This is because our immune cells are constantly being exposed to the microbes’ antigens without exposure to an adjuvant necessary for co-stimulation of that immune response; this repeated exposure (and lack of co-stimulation) results in our immune system saying, “hey, these microbes aren’t doing any harm, let’s flag these antigens so next time we see them we know not to bother reacting.”
Our gut also has immune cells called regulatory T lymphocytes (or Treg cells) that dull the immune response in this area. These Tregs secrete a protein called interleukin-10 (IL-10) which tells our white blood cells to stop presenting antigens. This is why a lot of vaccines have to be administered parenterally.
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Big picture is to differentiate what’s part of the body and what’s not.
Your digestive tract is essentially a tube stretching between your mouth and anus. For all intents and purposes, whatever happens in that tube is “outside” your body.
Your immune system wouldn’t engage gut bacteria in the same way it doesn’t engage any of the microbiome living on your skin.
If you have Netflix, watch Cells At Work. They have a really great couple of episodes about this very topic. Its very simply explained and super cute.😀
The gut is one of the most magical places in the human body. Normally, you’re right, the immune system’s job is to destroy anything inside the body that is not recognized as “self.” But there’s bacteria everywhere from your mouth to your anus. in some ways your gut is just a long tube that is technically “outside” the body. But we have to do some magic to make non-sterile food into something we can bring into the body. So the GI tract serves as this special zone where the immune system has to tolerate a lot of bacteria and other “not-self” stuff like food without going crazy and attacking as food gets broken down into things we can absorb. There are many complex mechanisms in place to keep the balance. It’s not so simple as not having an immune system in your GI tract as some comments here seem to imply. Side note, when this mechanism doesn’t work well and your immune system does go crazy when it sees a bunch of “not-self” stuff in the GI tract, that causes inflammatory bowel disease. It’s like the special gut system brakes are taken off leading to inflammation.
How do white blood cells not kill all of the bacteria on your skin?
You don’t constantly have blood on your skin, so you also don’t constantly have blood in your bowels.
Bacteria in your gut are only a problem when they start going where they aren’t supposed to.
Fun fact: Staphylococcus aureus, one of the causes of Staph infections and MRSA (methicillin-resistant staphylococcus aureus) is a normal part of the bacteria on your skin and in your upper respiratory tract (sinuses.) It’s only when s. aureus enters somewhere it isn’t supposed to be that causes a nasty infection.
There are two reasons.
One is that white blood cells don’t live in your gut.
The other is that the bacteria in your gut help train your immune system to identify friend and foe. So the “good bacteria” in your gut are not eliminated by the immune system that does exist there.
White blood cells aren’t in your gut. They’re in your blood.