you said "> if the thymus,
> or another organ did not do the job of sorting useful from useless and harmful, we
> would all be full of lymphocytes with no useful function at all."
I think you have forgotten (i) that programmed cell death is a major
player in the immune system, and that T cell maturation in the thymus
involves various selective events that rescue thymocytes from a preordained
death and (ii) it is likely also that thymic stromal cells are required to
sustain early thymocytes in the same way that B cell interactions with bone
marrow stromal cells (mediated via CD44/hyaluronate and c-kit/SCF
interactions) are required to sustain early B cells. It is therefore not
necessarily the case that lacking a functional thymus we would become
overloaded with thymic precursors.
On the question of what regulates T cell numbers, there is definitely a
genetic regulation, as T cell numbers in mice are strain dependent.
Various people (locally, Martin Raff, based on his observations on
differentiation of oligodendrocytes) have suggested that ALL cells are on
the point of death and are rescued by what one might call "anti-chalones"
or "survival factors"; increasing the number of cells titres out these
factors to the level where new cells fail to survive. Thus, the
concentration of these factors determines the total number of cells. In
Marty's system the relevant factor is NGF (I think, it's been a while since
I heard him talk last), so in some cases they are better defined than many
Paul J Travers phone : +44-(0)71-631-6262 (office)
ICRF Structural Biology Unit " " " 6268 (lab)
Birkbeck College fax : +44-(0)71-436-8918
London WC1E 7HX email : p_travers at icrf.icnet.uk
England or : paul at histo.cryst.bbk.ac.uk