Eli5 If the body is constantly renewing at a cellular level,why does the body reject organs or limbs from transplants?

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ie, if every cell is constantly replaced wouldn’t the organ eventually be replaced by your own cells that your isn’t trying to kill?

In: Biology

Like cells replace like cells. For example, your muscle cells are replaced with other muscle cells. So the cells of the organ are replaced with cells from itself, not cells from the host body.

The body is constantly renewing itself according to specific blueprints – plans that are unique to you, and your specific organs.

After receiving a transplant, your body is able to recognize that the organ in question isn’t part of the original blueprint. So it tries to get rid of it.

I think eventually is the operative word. Theybsay it takes 7 years to replace all cells give or take, its not actually teue as depends on cells etc

Long time to wait without a heart

Also the cells arent made elsewhere in the body and then deposited tonsay where the heart is, theyre made inside the heart which is continously renewing themselves, so a donar heart would just make the same cells and not “your” cells as it were.

The body has ways to determine whether a cell is an original part of it or not. If the cells of the organ in question do not have the same genes as the body in large then they run a risk of being attacked by the body’s immune system.

> if every cell is constantly replaced

New cells don’t just magically appear in tissue, hurled there from some central cell factory in the body. Cells divide to produce new cells, so for example new skin cells are going to be coming from the adjacent skin.

This means that a transplant is going to be growing new cells from its own old cells, retaining the DNA of the donor. It isn’t going to end up riddled with new organ cells with the DNA of the recipient. You can imagine this like human populations, if you transplant a community of ethnic Chinese people into the UK they won’t just randomly start popping out Caucasian babies from sheer proximity.

No it won’t, because the cell replacement happens locally, from stem cells specialised to that tissue – so for example the deep layers of your skin contain skin stem cells, that can duplicate and specialise into the different stem cells that make up your skin.

Almost all human cells have something called the major histocompatibility complex – a set of proteins on the surface of the cell that, like a sort of name tag or ID badge, tell your immune system to not attack your own body cells. This is determined by the DNA of the cell, and since a transplanted organ regenerates its cells from its own stem cells, it will continue to express the donor’s MHC, which, if it is too different from the host’s MHC, will cause the host’s immune system to attack the transplanted organ, leading to rejection.

The MHC proteins themselves are determined by multiple different genes, some of which exist in a few hundred different variations among the human population, so there is a huge number of different possible MHC configurations – that is why it is so difficult to find a matching partner for a transplant.