CD4+ T-cells have a number of different states, depending on whether they've been presented with an antigen or not and whether things like IL-6 were or were not present at the time. These various states can often be distinguished by cell-surface markers, so typically people will be able to flow sorts different cell populations from the blood. The ability to do this is one of the main reasons so many epigenetic studies are done on immune cells like this...they're easier to modulate and purify than things like neurons.