Defensin: A family of potent antibiotics made within the body by neutrophils (a type of white blood cell) and macrophages (cells that can engulf foreign particles). The defensins play important roles against invading microbes. They act against bacteria, fungi and viruses by binding to their membranes and increasing membrane permeability. On a chemical level, the defensins are small peptides unusually rich in the amino acid cysteine (Cys).The human defensins are classified into the alpha-defensins and beta-defensins on the basis of their sequence homology and their Cys residues.
From the Latin defendo, to repel. Defensin is also called human neutrophil peptide (HNP).