Interleukin 31 (IL-31) is a newly identified member of the gp130/IL-6 cytokine family that is composed of 144 amino acid and has molecular mass of 16 kD. IL-31 is produced mainly by T helper type 2 cells. The mature form of the protein contains a four-helix structure. Human IL-31 shares 31% amino acid sequence identity with mouse IL-31.The BioLegend ELISA MAX? Deluxe Set is a sandwich Enzyme-Linked Immunosorbent Assay (ELISA). A human IL-31 specific goat polyclonal antibody is first coated on a 96-well plate. Standards and samples are added to the wells, and IL-31 binds to the immobilized capture antibody. Next, a biotinylated goat polyclonal anti-human IL-31 detection antibody is added, producing an antibody-antigen-antibody “sandwich”. Avidin horseradish peroxidase is subsequently added, followed by TMB Substrate Solution, producing a blue color in proportion to the concentration of IL-31 present in the sample. Finally, the Stop Solution changes the reaction color from blue to yellow, and the microwell absorbance is read at 450 nm with a microplate reader.