A functional immune system requires the selection of T lymphocytes expressing receptors
that are major histocompatibility complex restricted but tolerant to self-antigens. This
selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a
surface receptor. In this review we summarize the current state of the field regarding the
natural ligands and molecular factors required for positive and negative selection and
discuss a model for how these disparate outcomes can be signaled via the same receptor …