IUBio

[Q] Transplantation Issues

Harold Bien hbien at jhunix.hcf.jhu.edu
Thu Mar 28 11:21:33 EST 1996


    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....)  



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