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?
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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.
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