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?


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

what exactly are you replacing?

if the original liver had to be removed for a transplant, the new organ is not using the former’s hepatocytes(liver’s precursor cells) because they are not present anymore.

whatever replacement these cells do is gonna be based on the donor’s cells.

First, that 3 year rule is basically nonsense. Cell turnover varies substantially between organs, tissues, and cell types. Second, the transplanted organ produces proteins that the host immune system identifies as foreign and attacks. Sure, the cells in the transplanted organ will divide and expand, but all the cells that are derived from the transplant contain the same incompatible proteins and will continue to be identified as foreign.

Let’s take a liver as an example. Each cell contains a string of DNA that acts as a recipe for various proteins. These proteins are what the host body sees as foreign and attacs, because it is produced from another DNA recipe.

When a cell “reproduces” it splits in two, each one keeping a copy if the original DNA. In effect the liver will keep the donor DNA for ever.

All cells that contain a nucleus have a Major Histocompatibilty Complex (MHC). This MHC is used to help your immune system recognize friendly cells (aka host cells) as well as a couple other things. When a transplant occurs, those donor cells contain the donor MHC as well. MHCs are more unique than fingerprints, so the odds of having an MHC similar enough to the host MHC so as not to trigger the immune system is… very unlikely.

There are mainly three possibilities with most transplants: hyper acute rejection, acute rejection, and chronic rejecting. The ideal transplant will be chronically rejected, meaning that the transplant will take years to no longer be accepted by the body. This can be done using immunosuppressive medicines to stop the body from rejecting it via the other two possibilities; this of course leaves the host with a weakened immune system and a life long prescription of immunosuppressants.

Source: made a B in Immunology

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.