Also, transplants from one identical twin to another are almost never rejected.
[ Source: Medline Plus ]
Why can a tissue from an identical twin be rejected ?
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MattDMo, I think you're right on. Also remember the thymus is where T-cell recombination and maturation occurs during fetal development, and rates are highest during neonatal period through pre-adolescent childhood - long after sharing a womb.
The immune system keeps developing during childhood. It's the immune system that's responsible for tissue rejection in transplant.
In our immune system, T cells attack and kill foreign cells in the body. They're supposed to ignore cells that carry molecules belonging to their own body. T cells go through selection in the thymus during these years to eliminate the T cells that attack their own molecules;the rest of the T cells that recognize foreign molecules are allowed to live.
Next, consider that environmentally-induced epigenetics influence the expression of proteins and other products. Even twins have different experiences and exposures, so there may be a difference in protein components in their bodies.
Also, this is conjecture, but the same concepts involved in autoimmune disorders (which are another topic entirely) might also be involved in some of these situations as well.
This was too long for a comment, but I have absolutely no evidence for what I'm about to say :)
It looks like that sentence was a little bit of hand-waving, as the author didn't want to categorically state that identical twin tissue would never be rejected. I suspect that in the early days there may have been reports of organ failure (esp. kidneys) in the recipient that were incorrectly identified as rejection episodes. I suppose there's also the slight possibility that one twin could have a mutation event very shortly after the separation of the twins in utero (when all the cells are still pluripotent) that could somehow change or (more likely) silence one or more of the HLA or other surface antigens involved in transplant rejection. This would almost certainly have to happen when the separated twin was a single cell. The mutated twin would therefore not express an antigen the non-mutated twin did, and if the non-mutated twin donated a kidney or other organ to the mutated one, its immune system wouldn't recognize the silenced antigen and could potentially result in rejection.
Like I said earlier, all pure conjecture :)