How do granulocytes and macrophages detect enemies?

John Richard Seavitt jrseavit at artsci.wustl.edu
Tue Oct 21 00:18:16 EST 1997


On 21 Oct 1997, Axel Boldt wrote:

> When compared to the incredibly beautiful mechanism of creating and
> employing specific antibodies, the above mechanism appears to be
> clumsy and not at all adaptable.

Let's first point out that that beautiful adaptive immunity takes some
time to develop..let's say three to seven days (depending on an assortment
of variables).  Becterial doubling time in rich media (ie, you) is about
twenty minutes.  Do the math, and you see the 'incredible evolutionary
pressure' for the host to evolve some method to delay pathogens until
specific immunity comes on-line (so to speak).
 
>  For example, I would expect there to
> be an incredible evolutionary pressure for the bacteria to avoid this
> finite set of features which macrophages detect. Why is this not so?

There is, and they do.  The number of ways pathogens evade the immune 
system is breathtaking.  There are some organisms which switch some of
their surface proteins according to a molecular clock between a set of
nearly-identical genes, to get the immune system to 'lose' its original
'target', if you will.  Others go to great lengths to evade being observed
by the 'surveillance' of the immune system.

However, some of the things that batercia in specific have that the immune
system targets (nonspecificly) are awfully useful to the bacteria.  For
some of them, alternatives do exist, but they are more complex systems
than the ones they replace, and thus more metabolically 'expensive'.
Remember that the evolutionary goal of a pathogen need not be the complete
evasion of the immune system, but rather only the evasion of it to the
degree that the organism can reproduce in sufficient numbers.  If you can
get away with using the 'cheapo' system, then that's fine.

John Seavitt




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