Defensins (alpha and beta) are cationic peptides with a broad spectrum of antimicrobial activity that comprise an important arm of the innate immune system. The α-defensins are distinguished from the β-defensins by the pairing of their three disulfide bonds. To date, six human β-defensins have been identified; BD-1, BD-2, BD-3, BD-4, BD-5 and BD-6. β-defensins are expressed on some leukocytes and at epithelial surfaces. In addition to their direct antimicrobial activities, they can act as chemoattractants towards immature dendritic cells and memory T cells. The β-defensin proteins are expressed as the C-terminal portion of precursors, and are released by proteolytic cleavage of a signal sequence and, in some cases, a propeptide sequence. β-defensins contain a six-cysteine motif that forms three intra-molecular disulfide bonds. BD-4 is expressed in the testes, stomach, uterus, neutrophils, thyroid, lungs and kidneys.