Figure 2.

Threshold tuning during gradual increase in TCR expression by DP thymocytes. Illustrated are selection boundaries for the signal intensity, induced in a DP thymocyte, as a function of TCR number. For higher levels of receptor expression, the average signal intensity (i.e., the baseline activity level) is higher, entailing higher activation threshold and viability-maintenance threshold. The thymocyte is not activated as TCR number increases if the fluctuating signal intensity (not shown) does not exceed the activation threshold. At higher intensities, negative selection occurs. If the intensity falls below the viability-maintenance threshold for too long, death-by-neglect may result.