As far as I can see, nobody has addressed the question of
*positive* selection of thymocytes. Negative selection can
be accomodated within the theory, but positive selection
violates the rules for thymocytes in Dr. Matzinger's article
in Annual Review of Immunology. It may allow danger signals
to be recognized at some later time, but when it occurs it
is self/nonself discrimination.
(I am responding by e-mail because my institution does
not carry Usenet).
Jack Komisar