Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate[1] and invertebrate[2] animals, plants,[3][4] and fungi.

An organism usually produces many different defensins, some of which are stored inside the cells (e.g. in neutrophil granulocytes to kill phagocytosed bacteria), and others are secreted into the extracellular medium.

[23] Other invertebrates known to produce defensins from this protein superfamily include molluscs, annelids and cnidaria.

[28] In addition to the defensins involved in host defence, there are a number of related Defensin-Like Peptides (DLPs) that have evolved to have other activities.

There appear to have been multiple evolutionary recruitments of defensins to be toxin proteins used in the venoms of animals;[29] they act via a completely different mechanism to their antimicrobial relatives, from binding directly to ion channels to disrupting nerve signals.

[29] Indeed, an insect defensin has been experimentally converted into a toxin by deletion of a small loop that otherwise sterically hindered interactions with the ion channels.

[33][34] In plants, a specialised family of DLPs is involved in signalling to detect if self-pollination has occurred and induce self-incompatibility to prevent inbreeding.

[36][37][38] In immature marsupials, because their immune system is underdeveloped at the time of birth, defensins play a major role in defense against pathogens.

Chun Kim et al. showed how anthrax, which produces a metalloprotease lethal factor (LF) protein to target MAPKK, is vulnerable to human neutrophil protein-1 (HNP-1).

In this sense, the intestinal production of antimicrobial peptides as hBD2 and hBD4 by trefoil from milk might play an important role on neonate colonization, thereby enhancing the immune response of newborns against pathogens with which they may come in contact.

At present, the widespread spread of antibiotic resistance requires the search and development of new antimicrobial drugs.

[51] Similar molecules, such as brilacidin, are being developed as antibiotics,[52] anti-inflammatories for oral mucositis,[53][54] and antifungals, especially for candidiasis.