Defensins are antibacterial, antiviral, and cytotoxic peptides of cationic nature which were isolated (and then sequenced) from the mammalian and bird neutrophils, some types of macrophages, and Paneth cells. Representatives of this peptide class are characterized by a variable number of the arginine and lysine residues as well as by the presence of six invariant cysteine residues forming intramolecular disulfide bonds. Defensins are active substances towards Gram-positive and Gram-negative bacteria, many fungi, and some enveloped viruses. The most possible mechanism of antibiotic effect of defensins consists in perforation of the target cell membranes and alteration of their barrier and metabolic functions. Defensins are able to play a many-sided role in immune responses of the organism: from a direct inactivation of various microorganisms during phagocytosis to a modulation of endocrinoimmune interactions. Defensins are evolutionary ancient, physiologically active substances participating in formation of immune responses in the organism.