The Human IL-4 ELISA Test Kit is designed for quantitative determination of the concentration of human interleukin 4 (IL-4) in serum, plasma, and cell culture supernatant. IL-4 is a protein of 129 amino acids that is synthesized as a precursor containing a hydrophobic secretory signal sequence. It is produced mainly in T cells. IL-4 is glycosylated at two arginine residues (positions 38 and 105) and contains six cysteine residues involved in disulfide bond formation. The disulfide bonds are essential for many biological activities. Some glycosylation variants of IL-4 have been described that differ in their biological activities. IL-4 enhances expression of class II MHC antigens on B cells. It can promote their capacity to respond to other B cell stimuli and to present antigens for T cells. IL-4 inhibits cell activation of NK-cells induced by IL-2 and stimulates the proliferation of thymocytes with the marker spectrum CD4 (-) CD8 (-), CD4 (+) CD8 (-), CD4 (-) CD8 (+).