The PC61.5 antibody is specific for mouse CD25, a 55 kDa surface protein also known as the Interleukin-2 Receptor alpha chain, or IL-2R alpha. CD25 may bind IL-2 by itself, although with low affinity and without induction of cell signaling. CD25 is also expressed within a high-affinity complex, along with the IL-2R beta chain (CD122) and the common gamma chain (CD132), to form a signaling receptor complex. Expression of CD25 varies during developmental stages of T and B cells, is induced on activated mature T and B cells, and is present on subsets of dendritic cells. CD25 signaling as part of the IL-2 receptor complex triggers T cell activation and proliferation, as well as modulating the differentiation and function of Th17 cells, T regulatory (Treg) cells, and dendritic cells.

The PC61.5 antibody is used as a marker for T cells, B cells and dendritic cell subsets. Expression of CD25, CD4 and the transcription factor Foxp3 is regarded as a phenotypic signature for Treg cells. As such, this antibody is widely used for depletion of Treg cells in vivo, as well as to distinguish Treg cells from naïve or conventional T cells which are CD25-.