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 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.
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 your body takes a lot less than 3 years to reject something. Obligatory extra sentence.
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