In article <36c67hINN29c at early-bird.think.com>,
Ian A. York <york at mbcrr.dfci.harvard.edu> 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
endogenous!
Or am I missing something in your question? :-)
_
o/ \\ // || ,_ o Mike Clark, mrc7 at cus.cam.ac.uk
<\__,\\ // __o || / /\, Cambridge University, Dept. Pathology
"> || _`\<,_ // \\ \> | "an antibody engineer who prefers the
` || (_)/ (_) // \\ \_ mountains"
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