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?

Question

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

Answer 1

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.