Positive and negative selection (T cell)

αβ T cells are subject to positive and negative selection prior to leaving the thymus. The goal is to allow the survival of T cells that can dock MHC but do not recognize self-peptide.

Positive selection
Cortical epithelial cells test whether a TCR can dock with a self-MHC protein.

Negative selection
Medullary epithelial cells select against self-reactive T cells by testing whether a TCR binds MHC/self-peptide too strongly.