Interleukin-17A (IL-17A, CTLA-8) is a CD4+ T cell-derived cytokine that promotes inflammatory responses in cell lines and is elevated in rheumatoid arthritis, asthma, multiple sclerosis, psoriasis, and transplant rejection. IL-17A is a 32 kDa long, disulfide-linked homodimer consisting of 136 amino acids that is a member of a six-species family of proteins (IL-17A-17F) and signals through the IL-17 receptor (IL-17R/CDw217). High levels of IL-17A homodimer are produced by activated peripheral blood CD4+ T-cells, and IL-17A also enhances expression of the intracellular adhesion molecule-1 (ICAM-1) in human fibroblasts. In particular, human IL-17A also stimulates epithelial, endothelial, or fibroblastic cells to secrete IL-6, IL-8, G-CSF, and PGE2. In the presence of human IL-17A, fibroblasts can sustain the proliferation of CD34+ hematopoietic progenitors and induce maturation into neutrophils. Mouse, rat, and human IL-17A can induce IL-6 secretion in mouse stromal cells, indicating that all homologs can recognize the mouse receptor. IL-17A regulates the activities of NF-kappa B and mitogen-activated protein kinases, stimulates the expression of IL-6 and cyclooxygenase-2 (PTGS2/COX-2), and enhances the production of nitric oxide (NO).