Living organisms are in perpetual contact with pathogenic microbes, and in encounter with parasites and predators.
In order to protect themselves, they produce a variety of antimicrobial proteins and peptides. One family of such protective
or defensive proteins is known as defensins, characterized by a cationic character, a low molecular mass, and an
abundance of cysteine residues. Defensins from mammals and plants have been succinctly reviewed by a number of experts
in this ever-growing field. This review encompasses the defensin plectasin from the saprophytic fungus Pseudoplectania
nigrella as well as defensins and defensin-like peptides from invertebrate animals such as jellyfish, sponges, nematodes,
crustaceans, arachnids, insects, bivalves, snails, and sea urchins. Big defensins from mollusks are mentioned together
with amphioxus big defensin. The structures and activities of these defense proteins are discussed.