Defensins are small (15-20 resiue) cysteine-rich cationic proteins found in both vertebrates and invertebrates. They are active against bacteria, fungi and enveloped viruses. They consist of 15-20 amino acids including six to eight conserved cysteine residues. Cells of the immune system contain these peptides to assist in killing phagocytized bacteria, for example in neutrophil granulocytes and almost all epithelial cells. Most defensins function by penetrating the microbial's cell membrane by way of electrical attraction, and once embedded, forming a pore in the membrane which allows efflux.