Defensins are cationic, anti-microbial peptides, produced by many cell types (e.g., leukocytes, epithelium, dendrites), and which play prominent roles in the innate immune response of mammals. Two classes have been described in humans, the alpha- and beta-defensins, which range from 3.5-4.5kD and are stabilized by three intramolecular disulfide bonds differing by the ordering of the disulfide bonds in the mature peptides. To date, six alpha-defensins (also referred to as Human Neutrophil Peptides, HNP1-6) and six beta-defensins (HBD1-6) have been investigated. All have demonstrated broad-spectrum in vitro antimicrobial activity against bacteria, fungi and enveloped viruses - activities that are expected to be significant for in vivo protection against pathogens. Researchers have also investigated the ability of defensins to induce the release from tissues of cytokines and chemokines involved in inflammatory and/or adaptive immunity, thereby providing a regulatory role that bridges innate and adaptive immunity. Investigative research has led to the measurement of defensins in serum, saliva, milk, amniotic fluid and lung and cervicovaginal lavage, and culture media of cells and tissue of blood, lung, skin, bowel, muscle, cartilage and kidney, among other sample types.