The MaxDiscovery™ Human IL-12 ELISA Test Kit is designed for quantitative determination of the concentration of bioactive, heteromeric form of human IL-12, p70, without interference by p40 monomer, homodimer, or IL-23 (p19/p40) in serum, plasma, and cell culture supernatant. Human interleukin 12 (IL-12), also known as natural killer cell stimulatory factor (NKSF) and cytotoxic lymphocyte maturation factor (CLMFIL-12), is a 75kDa glycoprotein heterodimer composed of two unequal, genetically-unrelated subunits. The smaller subunit (p35) and the larger subunit (P40). Each subunit of IL-12 apparently arises from a single copy gene. The transcription of the subunit of the subunit mRNAs is closely co-ordinated, although an excess of the larger subunit (P40) has been shown to be produced by B cells in addition to active IL-12. Expression of the smaller chain (p35) is reported to be enhanced by simultaneous expression of the larger chain (p40). Although IL-12 activity cannot be demonstrated in the absence of either chain, the presence of only p40 is associated with inhibition of IL-12-associated activities. As suggested by their names, p35 has a native molecular weight of 35 kDa while p40 has a native molecular weight of 40 kDa. In humans, p35 is a 22.5 kDa protein of 197 amino acids containing 7 cysteine residues plus 3 potential N-glycosylation sites and the molecule is believed to be heavily glycosylated. The p40 subunit is a 34.7 kDa protein of 306 amino acids containing 10 cysteine residues and four potential N-linked glycosylation sites. The murine p35 and p40 subunits show 60% sequence identity and 70% sequence identity to human p35 and p40, respectively. IL-12 is a pleitopic cytokine originally identified in the medium of cultured EBV-transformed RPMI-8866 cells. IL-12 is produced by monocytes/macrophages, B cells and connective tissue type mast cells. IL-12 shows species specificity with human IL-12 reportedly showing minimal activity in the murine system.