If most organ cells, like in the liver, are replaced every three years or so, why isn’t a transplant eventually accepted by the new body?
In: 350
Because your body takes a lot less than 3 years to reject something. Obligatory extra sentence.
because the donor liver cells replicate
if you loose your liver, and get a liver from someone else, you don’t have your own liver cells that could replicate, it’s the donor’s cells that replicate
in other words – new cells are created from old cells, if the old cells have donor’s DNA, then new cells will also have them.
the replacement cells dont come out of nowhere, they are born from the same cells they are replacing
For the same reason that you would let your own children into your house, but you’d query someone who merely “looked like” your child trying to enter your house.
Your cells are you. They are part of you. Grown by you. Their mother cells were… you. The child cells that they split into are you. They are you. The immune system recognises them as you, because if it was to attack them, it would literally be attacking you (that’s what an auto-immune disease is).
Your transplant… is still alive but clearly NOT you. It’s someone else. It comes from someone else. It reproduces, regenerates, makes new cells and grows. But its mother cells were someone else’s. Their child cells are not like you at all. It functions, but it’s not you.
Donors of certain body parts literally start to show two different DNA families – those of themselves and those of the donor part too. They are two entirely different organisms that have been lumped into the same body. They have just been made to co-operate by sewing them together, taking immuno-suppresants, etc.
So you have to take immune-suppressors to stop your body attacking all the “things that aren’t you” because it would attack the transplanted organ. If you stopped taking them, your body would literally try to purge that organ like it was a foreign invader. Because it basically is. It would treat them the same as an infection, a parasite, a virus, etc. because as far as it’s concerned that organ is not you. Even though it’s alive. Even though it’s attached. Even though it’s keeping you alive. It’s still the donor’s DNA.
So although your liver might reproduce, regenerate and replace its cells, it’s doing that with ITS OWN CELLS born from ITS OWN CELLS. The donor liver is doing that from ITS OWN CELLS born from ITS OWN CELLS. That don’t match the rest of your body’s cells and are seen as invaders.
Stop taking the immune-suppressors, and that organ literally dies within a couple of days because your body kills it.
Because no one seems to actually explain like you are 5:
Everything in your body has a little signature, an MHC major histocompatibility complex). These little signatures let your body know that that part of your body does in fact belong to your body. As your cells divide, and things are replaced, the new cells have the same signature, the same MHC.
When you get an organ transplant, the new organ has a different signature, a different MHC. Your immune system sees this new signature as “not you”, in the same way it sees a flu or a fungal infection as “not you”. Because the new organ is not you, your immune system tries to get rid of it. Even when the new organ makes new cells, it’s still telling those cells to make the same “not you” signature, so your immune system still tries to get rid of it.
Replaced is a bit of a misnomer. A million cells in your body die every second. The “replacement” cells are generated through mitosis, where a cell duplicates its DNA and separates itself in to 2 cells. So your body produces new cells from the previous existing cells, all containing your DNA. A transplanted organ does the same thing. Its cells duplicate their DNA and split in to 2 cells. The transplant continues to have its DNA, separate from your own, in perpetuity. Thus it will always be seen as a foreign object and won’t assimilate to your body.
My cells have my unique DNA that’s a combo from my mom and dad. Your cells have your unique DNA that’s a combo from your mom and dad.
Let’s say we want to transplant your liver into my body. Your liver is made up of your cells, and your DNA. My body is made up of my cells and my DNA. If we stick your liver in my body, my body will think your DNA is a foreign invader, and will attack.
It’s not exactly the same concept, but a good analogy would be thinking about why our body responds to a bacterial/viral infection. Our body recognizes that the bacteria/virus is “foreign” because the genetic material is different than the DNA in our own cells, and activates the immune system to attack these invaders. Similarly, my body will recognize that your cells are “foreign” because it’s not the same DNA, and will activate the immune system.
As an aside, this is why we want organs donors to be a “match”. The closer the donor matches the recipient, the less likely the recipient’s immune system will think the organ is an invader, which means the organ is less likely to get rejected.
Also, idk where you got the 3 year cell turnover rule, but that’s not really accurate. Certain cells (for example, the heart and brain), essentially never regenerate. This is why heart attacks and strokes are bad—those cells are dead and can’t come back. Meanwhile, skin cells and cells in our intestines regenerate pretty rapidly.
Outsider cells are replaced by more outsider cells.
The new organ’s own cells are the ones training their replacements, so to speak.
DNA are the “building blocks of life.” DNA lay out a series of instructions or “blueprints” to how we are made. No two human beings alive, even identical twins, have the same DNA. “Cells” are the smallest piece of life, making up living beings. That liver transplant makes up cells from another person whose DNA was not the recipient. Our bodies are smart enough to realize this. Our body attacks any new liver as a foreign element. To avoid this, you are put on medicine for the remainder of your life to suppress your body’s ability to do this. This also weakens you against sickness and disease.
Any replaced cells in a transplant organ are made by the foreign organ, not your own body. So they are just as rejectable as the original cells.
[I hadn’t ever heard the 3-year thing before, but even assuming it is true, it doesn’t really help at all here for the reason I just stated.]
Because the replacement cells are still made by the donor organ.
The problem with transplants is that host’s immune system recognizes them as foreign and wants to attack them. If Alice received a kidney from Bob, Alice’s immune system recognizes the cells in Bob’s kidney as not-Alice. When Bob’s kidney’s cells get replaced over time, it’s Bob’s kidney that’s doing the replacement. So all the new cells have “Bob” written all over them, and not “Alice”, because they were built based on Bob’s blueprints (DNA) and not Alice’s.
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