Defensins are small cysteine-rich cationic peptides containing three or four disulfide bridges. They are divided into families on the basis of the spatial distribution of their cysteine residues.Three families, the α-, β- and θ-defensins, can be distinguished in mammals. α- and β-defensins are characterized by antiparallel β-sheet structures stabilized by three disulfide bonds. The θ-defensins are found in rhesus monkey and some other non-human primates but not in human, chimpanzee and gorilla. They consist of two nine amino acid peptides derived from different precursor proteins joined by head-to-tail cyclization. Defensins protect human barriers from commensal and pathogenic microorganisms. Human α-defensin 6 (HD-6 with sequence AFTCHCRRSCYSTEYSYGTCTVMGINHRFCCL) is produced exclusively by small intestinal paneth cells. It is an antimicrobial peptide acting via 2 distinct antimicrobial mechanisms: HD-6 kills specific microbes depending on the local reducing environmental conditions, whereas known microbial trapping by extracellular net structures is independent of the reducing milieu.