Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have host defense properties, and are active against bacteria, fungi and many viruses. Beta-defensins are the most widely distributed, being secreted by leukocytes and epithelial cells of many kinds. This is a 5.1kDa 45-amino acid antimicrobial peptide called beta-Defensin-3 (hBD-3) having a beta sheet with three intramolecular disulfide bonds. It is expressed in high levels in keratinocytes and tonsilar tissue while expressed low in epithelia of the respiratory, gastrointestinal and genito-urinary tracts. Factors that induce its expression include TNF-alpha, IL-1beta and bacteria such as P. aeruginosa and S. aureus. hBD-3 is also potentially induced after exposure to IFN-gamma. In contrast to hBD-1, -2 and -4, hBD-3 demonstrates a salt-insensitive antimicrobial activity towards several pathogenic microorganisms at physiologic salt concentrations. This makes hBD-3 uniquely and particularly relevant in diseases where other hBDs show inactivity. The ability of hBD-3 to elicit its antimicrobial activity more effectively at the concentrations lower that those of hBD-1 and hBD-2 has been attributed to its amphipathic dimer structure and the increased positive surface charge (+9), compared to hBD-1 (+4) and hBD-2 (+6). hBD-3 has been shown to induce cytokine production from human keratinocytes and stimulates monocyte migration.