The MaxDiscovery™ Human IL-16 ELISA Test Kit is designed for quantitative determination of the concentration of human IL-16 in serum, plasma, and cell culture supernatant. Human interleukin 16 (IL-16), known as lymphocyte chemoattractant factor (LCF), was originally identified as a homotetramer consisting of individual 14 kDa monomers of 130 amino acids each. It is synthesized as a 68 kDa precursor (pro-IL-16) of approximately 631 amino acids lacking a signal peptide. The human IL-16 sequence shows more than 90% homology to those of nonhuman primates. It is a unique cytokine with no significant sequence homology to other well-characterized cytokines or chemokines. IL-16 is produced by epithelial cells, mast cells, lymphocytes, macrophages, synovial fibroblasts, and eosinophils. IL-16 mRNA is constitutively expressed in both CD4+ and CD8+ cells; however, transcription is induced in T lymphocytes upon exposure to antigen or mitogen. IL-16 may also be secreted by activated CD8+ cells in response to histamine or serotonin. IL-16 expression has been linked to inflammation processes in asthma, rheumatoid arthritis, systemic lupus erythematosus, colitis, atopic dermatitis, and multiple sclerosis. For example, the expression of IL-16 directly correlates with the number of infiltrating CD4+ T cells in asthmatic epithelium. IL-16 is a proinflammatory cytokine with chemotactic for CD4+ T lymphocytes, monocytes and eosinophils. In addition, IL-16 can upregulate IL-2 receptor and HLA-DR expression, inhibit T cell receptor (TcR)/CD3-dependent activation and promote repression of HIV-1 transcription. Recombinant pro-IL-16 polypeptides are specifically cleaved in CD8+ cell lysates suggesting that the actual secreted form of IL-16 may be smaller than the originally published 130 amino acid form. In CD8+ T cells, active caspase-3 cleaves pro-IL-16 producing a biologically active, secreted form of IL-16 (i.e. representing 121 C-terminal amino acid residues of pro-IL-16). Expression of CD4 is required for mediating IL-16 functions. Interaction between IL-16 and CD4 can specifically initiate an increase in intracytoplasmic calcium and inositol trisphosphate, activation of p56lck, and translocation of protein kinase C from the cytosol to the cell membrane. The region of CD4 that binds IL-16 has been identified within the D4 domain, overlapping the structure involved in CD4 dimer formation.