molecules (MHC Class I and MHC Class II) with different tissue dis$

Dr M.R. Clark mrc7 at
Fri Sep 30 08:25:54 EST 1994

In article <36c67hINN29c at>,
Ian A. York <york at> wrote:
>	[Ephraim asked about tissue distribution of MHC class I and II 
>molecules and Ian replied -]
>> 	Ephraim, this is due to the completely different functions of the MHC
>> molecules.  Since MHC class I scan the intracellular mileau for viruses 
>> or other intracellular parasites, which can hit most cell types, they 
>> need to be on most cell types.  The class II molecules scan extracellular 
>> media, so they are, in a sense, linked to anything else in contact with 
>> the extracellular medium (with distance limits, of course).  Therefore 
>> they can occur on intermittent cell types and still check the body as a 
>> whole.  
>[To which Ephraim replied]
>Thank you for your reply.  However, it begs the question of why not have 
>one class of MHC molecule that presents both endogenous and exogenous 
>antigens and which is expressed by all cells of the body?  Why should MHC 
>Class II be constitutively expressed by some cells (dendritic cells, B 
>cells, thymic epithelium) but be inducible on nearly all others?
[rest deleted]
Well an obvious reason not to have one type of MHC which presents both 
endogenous and exogenous antigens is that the T-cell recognising the 
MHC+peptide wouldn't be able to tell whether the antigen was exogenous or
Or am I missing something in your question? :-)
  o/ \\    //            ||  ,_ o   Mike Clark, mrc7 at
 <\__,\\  //   __o       || /  /\,  Cambridge University, Dept. Pathology
  ">    ||   _`\<,_    //  \\ \> |  "an antibody engineer who prefers the
   `    ||  (_)/ (_)  //    \\ \_   mountains"

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