An affinity ligand is reactive to the GARP protein may be capable of binding to an extracellular domain of GARP protein expressed on regulatory T (Treg) cells. The affinity ligand may be an antibody and may be used to identify Treg cells. A method comprises providing a blood sample from a subject and determining the amount of Treg cells in that sample. A composition containing Treg cells may be administered to an individual to suppress effector T cell activity in the individual. A composition containing an affinity ligand capable of binding to a GARP domain may be administered to an individual to suppress Treg cell activity and increase effector T cell activity in the individual. A kit for detecting Treg cells may include an affinity ligand reactive with mammalian GARP protein.