Defensins are a large family of broad-spectrum antimicrobial peptides, identified originally in leukocytes of rabbits and humans. Defensins, cationic/polar peptides (30-35 aa; 3-4kD), are distinguished by a conserved tri-disulfides and a largely b-sheet structure. Defensins, expressed at the cell surface, have been hypothesized to function as a biochemical barrier against microbial infection by inhibiting colonization of the epithelium by wide range of pathogenic microorganisms. In leukocytes, these peptides are stored in cytoplasmic granules and are released into phagolysosomes where they contribute to the killing of engulfed microorganisms. The genes encoding human alpha and beta-defensins are clustered in a contiguous segment of chromosome 8p23. Defensins are classified into two families designated alpha and beta based on distinctive, although similar, tri-disulfide linkages in the peptides. b-defensins are slightly larger and differ in the position and arrangement of 3 disulfides. In humans, six a-defensin (cryptidins), HD 1-6 (HD1-4 are also known as HNP1-4 for Human Neutrophil Peptides), and two b-defensins, HBD-1 and HBD-2, have been identified.