**2. General characteristics and classification**

Defensins are a family of antimicrobial peptides that form part of the innate and adaptive immune system and constitute the first line of host defense against microorganisms. It has shown the broad antimicrobial activity spectrum against bacteria, fungi and viruses [1, 24].

The defensins are small cationic peptides of 28–42 amino acids, characterized by a β-sheet structure linked by tree disulfide bonds, which are formed by six cysteine residues [2, 25]. Based on the distribution of their cysteines and disulfide bonds, defensins are classified into two groups in humans: α- and β-defensins [1]. α-defensins have 29–35 amino acids and the positions of the cysteines are C1-C6, C2-C4 and C3-C5, while the β-defensins are composed of 38–42 amino acids with the positions of the cysteines of C1-C5, C2-C4 and C3-C6 [26].

The α-defensins are expressed mainly in neutrophils and called human neutrophil peptides with four types (HNP1–4). There are other α-defensins (HD5 and HD6), known as enteric defensins, that are expressed in Paneth cells in the small intestine [27]. The expression of HBD6 has also been confirmed in the female genitourinary system [1, 28].

β-defensins are mainly expressed in epithelial cells throughout the body, including mucous membranes and skin [29–32]. Four types (HBD1–4) have been identified in humans, but several analyses indicate that there may be approximately 31 types, of which HBD5 and 6 are expressed in epididymis with their importance in defense against infections. The other defensins are known only for their antimicrobial activity [33, 34]. The β-defensin-1 is expressed constitutively, while the other three (HBD2–4) are expressed by the effect of proinflammatory cytokines or during the infectious process [1, 28].
