Editing Hysteresis (section)

===Immunology===

There is some evidence that [[T cell]]s exhibit hysteresis in that it takes a lower signal threshold to [[T cell activation|activate T cells]] that have been previously activated. [[Ras GTPase]] activation is required for downstream effector functions of activated T cells.<ref>{{Cite journal 
| last1 = Das | first1 = J. 
| last2 = Ho | first2 = M. 
| last3 = Zikherman | first3 = J. 
| last4 = Govern | first4 = C. 
| last5 = Yang | first5 = M. 
| last6 = Weiss | first6 = A. 
| last7 = Chakraborty | first7 = A. K. 
| last8 = Roose | first8 = J. P. 
| doi = 10.1016/j.cell.2008.11.051 
| title = Digital Signaling and Hysteresis Characterize Ras Activation in Lymphoid Cells 
| journal = Cell 
| volume = 136 
| issue = 2 
| pages = 337–351 
| year = 2009 
| pmid = 19167334 
| pmc =2662698 
}}</ref> Triggering of the T cell receptor induces high levels of Ras activation, which results in higher levels of GTP-bound (active) Ras at the cell surface. Since higher levels of active Ras have accumulated at the cell surface in T cells that have been previously stimulated by strong engagement of the T cell receptor, weaker subsequent T cell receptor signals received shortly afterwards will deliver the same level of activation due to the presence of higher levels of already activated Ras as compared to a naïve cell.