PRF1 and brain ischemia: To measure the impact of brain ischemia on NK cell activity and immune competence, we used flow cytometry to measure the expression of activation marker (CD69), maturation marker (CD27), functional receptors (NKG2D, CD158), and cytokines (Perforin, IFN-γ) in peripheral blood NK cells from human subjects with acute ischemic stroke and controls subjects.