The proposed results are significant, as the relative ratio of STAT3 and SHP-1 has been shown to be critical in T cell breast lymphoma and Hepatocellular Carcinoma pathologies. Our calculations suggest that if the STAT3T and SHP-1T concentrations are comparable, the phosphorylated STAT3 species increase as a function of the ratio of the forward phosphorylation reaction rate, kP, to the forward dephosphorylation rate, kD. The introduction of the kinase-protein and/or phosphatase-protein complexes enables additional regulatory capacity of the STAT signaling events. While the simplified model Fingolimod predicts earlier or later STAT activation on the relative kinase/phosphatase activity scale, the new model suggests additional regulatory steps taking place via modulation of the total amplitude. This result is critical from the immunological point of view, as it explains some aspects of the functional plasticity of T cell phenotypes. According to our model, T cell populations may undergo different transcriptional activation events in response to the same stimuli due to different kinase and phosphatase activity levels. Furthermore, since the kinase and phosphatase activities are subject to short and long term modulation, this gives rise to possible phenotype switching. It is critical to highlight that these effects can be described using the proposed detailed phosphorylation reaction model only. The range of the tested parameters suggests that the differences between this and the other models are due to the structure of the model rather than the parameters. Our analysis suggests that the STAT3 phosphorylation system with switch-like characteristics depends on the parameters of the model. The approach proposed by Goldbeter and Koshland is only applicable for limited physiological conditions when the concentration of JAKT and SHP-1T are significantly smaller than that of STAT3T. While these situations can occur in nature, most living cells exhibit comparable concentrations of enzymes and their substrates. Therefore, the physiological range of applications considered in is rather limited and all other phosphorylation events require the extended analysis described in this study. The model predicts the bell-shaped dependence for the intermediate species and provides a clear explanation as to how receptor-mediated activatory events can be followed by inhibition in response to the same signal. The model predictions for the multisite phosphorylation reactions obtained in this study are consistent with previously reported results. The model predictions in the form of bell-shaped curves for the intermediate phosphorylated protein species are consistent with the experimental data which suggests that IRF-5 requires phosphorylation of at least two sites for activation. We compared our model predictions with the previously reported method of Goldbeter and Koshland. There are two key biochemical factors that may significantly vary in living cells and thereby affect the signaling properties: the ratio of total protein to kinase and phosphatase concentrations and the rates of phosphorylation, kP, and dephosphorylation, kD, reactions. For simplicity, we did not vary the phosphatase concentration and changed the kinase activity only. Our analysis shows that alterations of phosphorylation rates and total IRF-5 to AP ratios do not have any impact on the model in. Figure 4 shows the range of the model prediction for the described variation of parameters. It can be seen from the Figure 4 that for comparable phosphorylation to dephosphorylation rates, the non-phosphorylated form of IRF-5 appears to be dominant.