Schematic representation of ELISAs for detection of human IL-15. (A) Total IL-15 was detected by a commercially available IL-15 ELISA (R&D Systems, Quantiglo kit). The capture and detection monoclonal anti–human IL-15 antibodies target different accessible regions of IL-15 molecule. (B) IL-15/IL-15Rα heterodimers were detected using a monoclonal anti–human IL-15 as capture antibody and a polyclonal anti–human IL-15Rα as detection antibody. Single-chain IL-15 or purified sIL-15Rα tested separately were not detected by this heterodimeric IL-15/IL-15Rα ELISA.