Definition of POSITIVE SELECTION

FORSDYKE at QUCDN.QueensU.CA FORSDYKE at QUCDN.QueensU.CA
Sun Jan 30 16:35:19 EST 1994


  Lee Wetzler suggested POSITIVE SELECTION as one of the "top ten"
developments in Immunology in the past 15 years. To avoid misunderstand-
ings, it might be useful  to get some agreement on what we mean by this
term.

    SELECTION. In biology the word "selection" implies discrimination.
One may indeed select one chocolate from a box inwhich all chocolates are
identical, but here there is no discrimination. Selection in a biological
context means going to a box containing an assortment of different
chocolates and picking one out on the basis of some criterion ( I usually
go for the biggest).

    POSITIVE. If you fish in a "pool" with a net of a certain mesh then
you positively select fish which are bigger than the mesh size to take
home to market. Fish smaller than the mesh size are simply not affected.
They are NOT negatively selected against.

    NEGATIVE. If you take the fish you have caught in the above net and
destroy them, then you have removed them from the available pool of fish.
Someone fishing subsequently with a net of smaller mesh than yours will
catch fish to take to market, but these will be of intermediate size
defined by the difference between the mesh sizes. Thus, because of prior
negative selection the available repertoire of fish sizes is diminished.
Negative selection is not the absence of positive selection.

This is pretty basic stuff. So perhaps all are in agreement?

                 Sincerely, Don Forsdyke,
                            Discussion Leader. Bionet Immunology.



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