Why MHC Class I and II molecules?

Mark Hodes markhodes at aol.com
Sun Sep 25 23:16:05 EST 1994

In article <36584i$306 at jhunix1.hcf.jhu.edu>, ejf at welchlink.welch.jhu.edu
(Ephraim Fuchs) writes:" Does anyone wish to speculate why there should be
two types of MHC 
molecules (MHC Class I and MHC Class II) with different tissue

If evolution has taught us anything, it is that organisms are kludges. 
Ted H. Hansen, et al in Paul (Fund. Imm. 19943) lists the following
immunological properties associated with the MHC:
1 vigorous rejection of tissue grapfts
2 stimulus of Ab production
3 stimulation of MLR
4 GVH reactions
6 Ir genes
7 restriction of immune response
8 Ag presentation to T cells
Even excluding 1 & 4 as adaptations leaves diverse properties
 which may have "piggybacked" when adaptations serving one
 function were exploited as preadaptations for others. 
 But then gene duplication coupled with differential 
suitability to task could fix genetic divergences until 
separate MHC types evolved.  Differential suitability to task
may be exemplified by MHC I molecules presenting peptides from
endogenously derived proteins and MHC II exogenous proteins.

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