Why will MHC indicate graft as foreign?

[Ian A. York]

In article <noemail-325A33.15594108072003 at cnews.newsguy.com>,
Hobo  <noemail at noemail.com> wrote:
>
>Why don't NK cells kill red blood cells, which don't express MHC Class 
>Ones, and why don't red blood cells express MHC Class Ones? Is it 
>because without a nucleus MHCs are not needed, or is because without a 
>nucleus they can not be produced?

Low surface MHC class I is not a signal for killing by NK cells; rather, 
high surface MHC class I is a signal not to kill.  NK cells also require 
positive signals to kill target cells.  If those positive signals are 
absent (as is presuambly true on RBCs) then it doesn't matter if there's 
no negative signals; the NK cells will not kill anyway.  (Oversimplified 
but generally accurate, I hope.)

I know of no viruses that infect RBCs (in theory, I suppose, something 
like a poxvirus might be able to infect even an anucleate cell), and 
anucleate cells won't be cancerous per se (thoguh they might be the 
product of cancerous progenitors).   Those are the main reasons for which 
MHC I is required.

Ian 
-- 
    Ian York   (iayork at panix.com)  <http://www.panix.com/~iayork/>
    "-but as he was a York, I am rather inclined to suppose him a
     very respectable Man." -Jane Austen, The History of England