The present invention relates to a method, in particular an in vitro method, for identifying PD1+ cells, comprising analyzing the methylation status of at least one CpG position in the mammalian gene region for Programmed cell death 1 (PDCD1), wherein a demethylation or lack of methylation of said gene region is indicative for a PD1+ cell, when compared to a non-PD1+ cell. The analyses according to the invention can identify PD1+ cells on an epigenetic level and distinguish them from all other cells in complex samples, such as, for example, other blood or immune cells. The present invention furthermore provides an improved method for quantifying PD1+ cells, in particular in complex samples. The method can be performed without a step of purifying and/or enriching cells, preferably in whole blood and/or non-trypsinized tissue.