The present invention relates to a method for detecting regulatory T cells (Tregs) in the blood of a subject by use of a novel marker. The method includes the detection of at least one commonly known marker for regulatory T cells (Tregs) in combination with the detection of the marker BTNL8. The method allows the reliable detection and quantification of different Treg populations which are involved in important immunological functions, such as immunosuppression and maintenance of immune tolerance. The invention also provides a method for monitoring the treatment of a subject suffering from an immunological disease or condition which makes use of the novel marker. The invention also relates to the use of a means that allow the specific detection of BTNL8 for detecting Tregs. Kits comprising detection means for detecting commonly known markers of Tregs along with BTNL8 are also provided.
