Since the responses have been quantized to two values, on and off, and there are three epitopes, the total number of possible states of the system is 2 3 ¼ 8. Figure 6b depicts the state transition table obtained from the transition rules in Figure 6a . Using this table, one can determine the attractors of the system, which are the states or sets of states in which the system stays in the long run in the absence of external disruptions. For each attractor, there is associated a basin of attraction, containing attractors and transient states, which are the sets of states that lead to the attractor [108] . In the context of biological systems, attractors are a mathematical model for homeostasis. In our minimalist example, we see that there are two different behaviors, corresponding to the two distinct basins of attraction in Figure 6c ; the respective attractors are indicated by dashed rectangles. As can be seen, which of the two behaviors the system is in depends only upon the state of the response to epitope A. If it is off (there is no help), then the system may pass through some irrelevant transient states but it will always tend toward the resting single-state 000 attractor and thus an absence of activity; see the diagram on the left in Figure 6c . If the response to epitope A is on (there is help), then the activity of the system corresponds to that of a cyclic attractor, with the effector DOI: 10.1371/journal.pcbi.0020081.g006 Figure 6 . Example of a Simple Immunomic Network, Consisting of Three Epitopes Epitope A is a promoter (A is specific to CD4 þ helper T cells), epitope B is a suppressor (B is specific to CD4 þ regulatory T cells), while epitope C produces the effector response (C is specific to CD8 þ cytotoxic T cells), while also promoting the suppressing response of epitope B (negative feedback). response being turned on and off cyclically; see the diagram on the right in Figure 6c . This situation corresponds to modulation of the effector response and regulation of the inflammatory response by means of a negative feedback mechanism. Note that when there ceases to be a response to epitope A, the system jumps to the other basin of attraction, and tends to the resting 000 state. The immune response to epitope A in this example determines the behavior of the system, and thus it functions as the master of the overall immunological response, with the individual immune responses to epitopes B and C being slave to it. The concept of master-slave regulatory units is quite important for the understanding of complex regulatory systems and has in fact been considered as a mechanism for genomic regulation (Michael Bittner, unpublished data).