I'm sorry if these are very basic transplantation issues, but after
attending a lecture on tissue engineering, I am left with several questions.
First, tissue engineering is the branch of science which is trying to
grow, in vitro, human tissue in such a way that they may be applied to a
living human. For instance, in burn victims, their burnt skin is usually
removed and the wound covered with cadaver skin. In tissue engineering,
the approach would be to cover with "genetic" live skin.
My question is how come, according to the lecturer, the epidermis
would get rejected (ie. the cadaver skin), but the dermis, the substitute
skin, would not? What enables the skin to be exempt from the
transplantation issues?
Her answer was that since the dermal cells (endothelium I believe)
do not express a large number of surface antigens, and the fact that they
do not express MHC II antigens, they are able to "escape" from
rejection. Also, she stated that because the cells are built on a
scaffold of cellular matrix (I forget exactly what it was), the immune
system seems to be tolerant. In fact, experimentally, they have yet to
experience a rejection.
If this is in fact the case, then it seems that there are serious
flaws in our immune system. How can the immune system distinguish
between "human" and "non-human" material? The issue is simple if the
immune system simply attacked any "non-self" material, but if it is
tolerant of other "human" or "human-like" materials, then how is this
determination made, especially if the cells express little or no surface
antigenetic markers?
I have a feeling that I'm missing something. But even then, what
about pig heart valves. Why do pig heart valves escape rejection?
Especially since they lie in a bath of blood filled with leukocytes. I
can understand if perhaps the flow rate was so high such that the
leukocytes didn't have an opportunity to engage the pig valve, but this
seems riduculous as after a period of time, statistically speaking,
chance will catch up and an immune reaction should occur.
If I were to recieve an infusion of a HUMAN blood with additional ABO
antigens (say I'm O), my system will respond against the foreign rbcs. I
bet if I were to receive an infusion of PIG blood, my blood would clot in
my first transfusion. Therefore, if the immune system can be so specific
in this regard, why then does it allow pig valves in our hearts,
"generic" foreign dermal tissue, and tissue engineered cartilage?
Thanks for any information regarding these basic questions....
(I'm sure the answers are in some basic immunology text, but I'm a bit
lazy and am hoping that someone more knowledgable may be able to inform
me without much pain....)
