MHC molecules

[mark]

ct17 wrote:
> 
> T-cell receptors can only recognize Ag. that are assoc. with cell membrane
> proteins(MHC I-II).  My question is  Where does the MHC molecule originate ?
> 
> reply to :     ct17 at erols.com
> Dean

Dean, Assumeing you mean what cell, its the APC (dendritic cells and 
their cousins like langerhans cells in skin).  MHC molecules get 'loaded 
with peptide fragments from degraded proteins in cytosol.  the peptide 
fragments are generated by the proteosome which might be considered as 
the trash compactor of the cells.  Ubiquinated and non-ubiquinated 
proteins may be handled differently.  WHy this situation exists is due to 
the education of t cells.  THymic education takes place on cells that 
also express the MHC, and only T cells with receptors that bind the 
expressed MHC will be able to survive the slings and arrows of outrageous 
thymi.
markH