Figure 6

Non linear extracellular feedback: a design principle that can balance speed and accuracy of a recognition system. Consider a system of cells that can be activated by recognition of a ligand. Each cell responds at a different level, depending on its specificity, and recognition threshold for cell activation is set through a positive feedback. Cells search for ligand and are thus activated in a random timing. We compare three cases: an intracellular feedback, a linear extracellular feedback and a non-linear extracellular feedback. In the case of an intracellular feedback (no intercellular communication), the system’s response is fast as every cell that passes the recognition threshold is activated. However, the response will be less specific, since all cells above threshold respond, not only the strongest ones (lower left point). In the case of a linear extracellular feedback, the strongest cell always wins, regardless of the time of its activation. In this case specificity of the system’s response is high, but time is lost until the strongest cell is activated, thus the response can be slow (upper right point). A non-linear extracellular feedback balances this tradeoff by opening a window of opportunity for competition. The strongest cell that arrives during this time window will win. The length of this time window is tunable, and depends on the relative activation levels of the interacting cells.