T Cell Selection

• Development of a functional T-Cell Receptor (TCR) in thymus

1. Positive Selection (Thymic Cortex)

First, cortical thymic epithelial cells (cTECs) present antigens on MHC to T Cells. The cells that bind well within 3-4 days receive survival signal (positively selected), while cells that do not bind or bind weakly (low affinity) do not receive survival signal and die (~90% of cells)

• Those that bind well to MHC I become Cytotoxic (CD8+) (💡1*8 = 8)

• Those that bind well to MHC II become Helper (CD4+) (💡2*4 = 8)

2. Negative Selection (Thymic Medulla)

The medullary thymic epithelial cells (mTECs) present antigens from self tissues on MHC I (CD8+). Since MHC II cannot present self-antigens, dendritic cells phagocytize mTECs and then express self-antigens on MHC II (CD4+). 

• Cells that bind too strongly receive an apoptotic signal

  • Ensures that T cells do not overreact to self-antigens (causes auto-immune disease)

Test Yourself: