Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants. They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino acids including six (in vertebrates) to eight conserved cysteine residues. Cells of the immune system contain these peptides to assist in killing phagocytosed bacteria, for example in neutrophil granulocytes and almost all epithelial cells. Most defensins function by binding to the microbial cell membrane, and, once embedded, forming pore-like membrane defects that allow efflux of essential ions and nutrients.