  Our main tenet argues that the primary role of positive thymic selection and
the resulting T cell population is the maintenance of a homeostatic equilibrium
with self MHC-self peptide complexes. The homeostatic T cell repertoire can
recognize infections non-specifically and this is an indirect (negative)
recognition: the whole homeostatic T cell population together "holds a mirror"
to the whole self, and any MHC-peptide complex that is "not reflected in the
mirror" can be perceived by surrounding homeostatic T cells as a signal of the
presence of a foreign entity. On the other hand, infection-specific T cell
clones arise in a different pathway in the periphery, do not enter the thymus,
and form a functionally different population. Here we summarize the basic
assumptions and consequences of a logic-based new model, which differs from
conventional models in many respects.
