Antimicrobial peptides are produced by plants and most organisms throughout the animal kingdom including humans. Antimicrobial peptides protect against a broad range of infectious agents, as bacteria, fungi, and viruses. The amphibian skin is an especially rich source of antimicrobial peptides. See also the product families: Hepcidins LL-37 and Fragments Tuftsin and Analogs (subfamily).

Defensins comprise a family of small, cationic peptides prevalent in neutrophil granulocytes of humans, rabbits, guinea pigs and rats as well as in rabbit lung macrophages. These peptides show sequence homology and contain 6 conserved disulfide-linked cysteines. In vitro, all defensins display a range of prominent antimicrobial activities against bacteria, fungi, and certain enveloped viruses. In addition, human and rabbit defensins exert potent cytotoxicity in vitro against various mammalian tumor cells. The defensins act on their targets by permeabilizing the plasma membranes.

The theta-defensin retrocyclin-1, also called RC-100, is a circular octadecapeptide containing an internal trisulfide ladder. Besides its antimicrobial activity, retrocyclin-1 has a remarkable ability to protect human cells in vitro from infection by T- and M-tropic strains of HIV-1. Its effectiveness against HIV-1 results from the prevention of viral entry, and is correlated with its ability to recognize and bind with high affinity to glycoproteins and glycolipids implicated in HIV-1 entry, such as gp120 (KD = 35.4 nM), CD4 (KD = 31 nM), and galactosylceramide (KD = 24.1 nM). Retrocyclin represents a new class of small molecule HIV-1 entry inhibitors.