Interesting questions Jim--
I'm not an evolutionary biologist, but I'll take a crack at this. It seems
pretty clear that humoral immunity evolved from a more generic form of
cell adhesion since so many of the latter molecules have the
"Ig-superfamily" fold structure seen in Ig. Moreover there is a certain
jury-rigged, Panda's thumb-ish aspect to the Ig singnaling mechanism,
i.e. the signal is not transduced directly, instead being sent via Ig-a/b.
This mechanism may have arisen because (teleologically speaking) Ig (and
the TCR) have such puny cytoplasmic domains that they cannot adequately
interact with kinases, etc., let alone send discrete signals indicating
which of the four sIg types has been cross-linked. And it's asking alot of
the tiny Ig-a and Ig-b to do this job of signal discrimination...
Of course increasing the number of different Igs on a B cells would
presumably decrease the numbers of each type produced and expressed,
lowering the overall affinity of that cell for its cognate antigen and
increasing the required antigen load needed to activate the B cell (this
last may be a trivial difference. I'm a biologist, not a biochemist/
Larry Pike-Nobile ez063669 at ucdavis.edu
Arms folded/To the moon/Among the cows-J.Kerouac
>>On Tue, 9 Dec 1997, Jim Kling wrote:
>>> All true -- but what if, along with being able to present two types of
> antibody, the B cell had two transmembrane signalling cascades, one for
> each antibody? That would allow it to distinguish which antibody had
> been activated.
>> I suspect the reason it hasn't worked out this way may have as much to
> do with the process by which humoral immunity evolved. Any evolutionists
> out there want to take a crack at the question?
> Jim Kling
> Writer and Consultant
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