T-cell model. (A) Structure of the Boolean model as shown in CellNetAnalyzer . (B) Hill functions with parameter n = 3 and different thresholds k = 0.3, kfast = 0.1 and kslow = 0.8. (C) Subnet of the T-cell model for scenario 2 (only activation) and scenario 3 (activation and feedback). (D) Numeric simulation of the subnet from (C). Time is plotted on a log-scale. The two dotted vertical lines indicate the time points when the concentration of ZAP-70 exceeds kfast and kslow, respectively.