defensine

Defensine, or defensin, refers to a family of small, cationic, cysteine-rich antimicrobial peptides that form part of the innate immune system across many organisms. They typically range from about 18 to 45 amino acids in length and are stabilized by three intramolecular disulfide bonds. Defensins are classified into several families, most notably alpha-, beta-, and theta-defensins, with plant defensins as a distinct, broadly studied group.

In humans, alpha-defensins include HD5 and HD6, primarily produced by neutrophils and Paneth cells; beta-defensins (BD-1 to BD-4) are expressed by epithelial cells in the skin and mucosal surfaces. Theta-defensins occur in some non-human primates and are generally not expressed in humans. Plant defensins are found in many plant species and contribute to protection against pathogens.

Mechanism of action: Defensins exert antimicrobial effects by interacting with microbial membranes due to their positive charge, leading to membrane disruption and pore formation. Some also modulate the host immune response by acting as chemoattractants for immune cells and by influencing cytokine production and wound healing.

Expression and regulation: Defensin gene expression can be induced by microbial components, inflammation, and chemical signals. They are stored in neutrophil granules or produced by epithelial cells and secreted at mucosal surfaces or into extracellular spaces where they can act on microbes.

Clinical relevance: Defensins contribute to barrier immunity and have been studied for therapeutic potential as antimicrobial agents or vaccine adjuvants. Altered defensin expression has been linked to susceptibility to infections and to inflammatory conditions such as Crohn's disease and psoriasis. Ongoing research explores their roles in microbiome modulation and disease.