In this paper a group of proteins, defensins, is characterized as they seem to be a very important element of immunity, especially innate immunity. Twelve alpha-defensins (HNP1-4, HD5-6, NP1 and 5 theta, criptidin 3 and 4, RMAD3-4), 25 beta-defensins (HBD1-4, SBD1-2, TAP, LAP, EBD, BNBD1-13, PBD1-2, mBD-1), and 5 q-defensins (retrocyklin 1,2, RTD1-3) are described. They display biological roles which are not only antibacterial, but also antiviral, antiparasitic, and antifungal.