The decisional process for the LEUKOCYTE MEMBRANE and each MEMBRANE UNIT during a simulation cycle. (A) The LEUKOCYTE steps through its decisional process only once during a simulation cycle. At the start of the cycle, the MEMBRANE instructs all MEMBRANE UNITS within the CONTACT ZONE to follow the decisional process in B. Once that process is complete, the MEMBRANE completes its process by selecting and following the one applicable action option. (B) A MEMBRANE UNIT is described and illustrated in Figure 2. The state of each depends on the properties of the three LIGAND objects contained within. During each simulation cycle, each MEMBRANE UNIT, selected at random, uses this decisional process to update its status relative to the SURFACE UNIT over which it is positioned. (C) The hierarchical organization of the ISWBC system is illustrated. There are six levels. An Experiment Agent exists within the system, but separate from the FLOW CHAMBER and LEUKOCYTE. It represents a researcher conducting wet-lab experiments: it measures and records events and behaviors during each simulation.