Defensins are cysteine-rich cationic antimicrobial peptides produced primarily by neutrophils, intestinal Paneth cells, and various mucosal epithelial cells. They can directly disrupt microbial membranes, entrap pathogens, recruit immune cells, and regulate the mucosal microbiota, making them important effector molecules of the innate immune barrier.

Keywords: defensins; α-defensins; β-defensins; HNP; HD5; HD6; antimicrobial peptides; mucosal immunity

1 Molecular Basis of the Defensin Family

1.1 Basic Characteristics of Defensins

Mature defensins generally consist of approximately 29–45 amino acids and contain six conserved cysteine residues that form three intramolecular disulfide bonds. These disulfide bonds stabilize the short peptides in a β-sheet conformation, allowing them to maintain their structure in protease-rich environments such as neutrophil granules, the intestinal lumen, and mucosal secretions. Most defensins carry a net positive charge and can recognize negatively charged bacterial membranes, fungal membranes, or viral envelopes through electrostatic interactions.

1.2 α-Defensins and β-Defensins

Human defensins are primarily classified into α-defensins and β-defensins. The two groups share similar three-dimensional scaffolds but differ mainly in cysteine spacing, disulfide-bond connectivity, tissue origin, and maturation processes.The disulfide bonds of α-defensins generally connect Cys1–Cys6, Cys2–Cys4, and Cys3–Cys5. This group includes the neutrophil-derived HNP-1–4 and the Paneth cell-derived HD5 and HD6. The disulfide bonds of β-defensins generally connect Cys1–Cys5, Cys2–Cys4, and Cys3–Cys6. β-Defensins are produced mainly by epithelial cells in the skin, respiratory tract, gastrointestinal tract, and urogenital tract.

Table 1. Major classes of human defensins

1.3 Precursor Synthesis and Maturation of Defensins

Defensins are generally synthesized as precursor proteins containing a signal peptide, a propeptide region, and a mature peptide region. The signal peptide directs the peptide into the secretory pathway, whereas the propeptide region neutralizes the positive charge of the mature peptide and prevents defensins from inappropriately attacking host-cell membranes.Neutrophil α-defensins are processed during granule formation. Intestinal α-defensins require tissue-specific proteases for maturation. Trypsin participates in HD5 processing in the human intestine, whereas the maturation of mouse Paneth cell α-defensins mainly depends on MMP7.

2 Major Members of the α-Defensin Family

2.1 HNP-1, HNP-2, and HNP-3

HNP-1, HNP-2, and HNP-3 are highly abundant α-defensins in human neutrophils. Their sequences are highly similar, with the principal differences occurring at the N-terminus. HNP-2 can also be generated through N-terminal processing of HNP-1 or HNP-3.

After neutrophils engulf pathogens, HNP-containing azurophilic granules fuse with phagosomes. Neutrophil degranulation, neutrophil extracellular trap formation, and cellular injury can also release HNPs into the extracellular environment.

2.2 HNP-4

HNP-4 differs substantially in sequence from HNP-1–3 and is present at relatively low levels in neutrophils. Nevertheless, it exhibits strong activity against certain Gram-negative bacteria.

Individual HNP members differ in membrane-binding capacity, salt sensitivity, and antimicrobial spectrum. Total HNP levels reflect the overall release of neutrophil α-defensins but cannot replace the quantitative and functional analysis of individual members.

2.3 HD5 and the Intestinal Antimicrobial Barrier

Human defensin 5 (HD5) is produced mainly by small-intestinal Paneth cells and stored in secretory granules. Bacterial components, cholinergic stimulation, and tissue injury can promote its release.Mature HD5 can bind bacterial membranes and alter membrane permeability. It directly inhibits various bacteria and some fungi and can also restrict microbial access to the intestinal crypt stem-cell compartment.

2.4 HD6 and Pathogen Entrapment

Human defensin 6 (HD6) has relatively limited direct membrane-disrupting activity. Its principal characteristic is the ability to self-assemble into fibrous nanonets on pathogen surfaces.These nanonets can envelop bacteria and prevent their adhesion to or invasion of epithelial cells. The entrapped pathogens can subsequently be cleared through intestinal peristalsis and mucus renewal.

3 Major Members of the β-Defensin Family

3.1 HBD-1: Constitutive Epithelial Defense

Human β-defensin 1 (HBD-1), encoded by the DEFB1 gene, is constitutively expressed at relatively stable levels in epithelial tissues of the kidneys, respiratory tract, gastrointestinal tract, and urogenital tract.The activity of HBD-1 is influenced by the redox environment. Reduction of its disulfide bonds changes its conformation and exposes hydrophobic regions, potentially producing stronger activity against certain anaerobic bacteria.

3.2 HBD-2: An Inflammation-Inducible Defensin

Human β-defensin 2 (HBD-2) has low basal expression in most tissues. Bacterial products, IL-1β, TNF-α, and other inflammatory signals can induce its expression through pathways such as NF-κB and MAPK.In addition to directly inhibiting microorganisms, HBD-2 can recruit CCR6-positive immature dendritic cells and memory T cells, connecting innate epithelial defense with adaptive immune responses.

3.3 HBD-3: Broad-Spectrum Antimicrobial and Immunoregulatory Activity

Human β-defensin 3 (HBD-3) has a high net positive charge and is active against various Gram-positive bacteria, Gram-negative bacteria, and some fungi. It can retain a degree of antimicrobial activity under relatively high ionic-strength conditions.HBD-3 can also regulate signaling in dendritic cells, monocytes, and keratinocytes. These effects are strongly dependent on concentration and tissue context.

3.4 HBD-4 and Other β-Defensins

HBD-4 is expressed mainly in epithelial tissues of the respiratory, gastrointestinal, and urogenital tracts and can be upregulated following infectious or inflammatory stimulation.Members such as HBD-5 and DEFB118 exhibit greater tissue selectivity and are mainly associated with local antimicrobial defense in the urogenital tract, protection of the sperm surface, and maintenance of reproductive tract immune homeostasis.

Table 2. Functional comparison of major human defensins

4 Antimicrobial Mechanisms of Defensins

4.1 Electrostatic Recognition and Accumulation on Microbial Surfaces

Lipopolysaccharides, teichoic acids, and acidic phospholipids on bacterial surfaces generally carry negative charges and can attract positively charged defensins. After defensins accumulate on the microbial surface, their hydrophobic residues approach the lipid bilayer, creating favorable conditions for membrane insertion, peptide aggregation, and lipid rearrangement.The outer leaflet of mammalian cell membranes generally has a lower density of negative charges, providing defensins with a degree of selectivity toward microbial membranes.

4.2 Disruption of Membrane Permeability

Some defensins can insert into microbial membranes and oligomerize, causing thinning of the lipid bilayer, transient membrane defects, or the formation of ion-permeable pores. These changes ultimately lead to membrane-potential dissipation and leakage of intracellular contents.Depending on the defensin and the composition of the microbial membrane, the process may involve pore formation, surface coating, lipid clustering, or nonspecific membrane disruption. Not all defensins should therefore be described uniformly as membrane pore-forming peptides with a fixed pore structure.

4.3 Interference with the Cell Wall and Intracellular Processes

Some defensins bind bacterial cell-wall precursors, lipopolysaccharides, and other membrane-associated molecules, interfering with cell-wall synthesis and outer-membrane stability.

Defensins that enter microbial cells may also affect DNA, RNA, protein synthesis, and enzymatic activity. Their antimicrobial effects are therefore not limited to plasma membrane disruption.

4.4 Pathogen Entrapment and Aggregation

HD6 can form nanonets on pathogen surfaces, while certain other defensins can promote bacterial aggregation. These processes restrict microbial motility, epithelial adhesion, and tissue invasion.

Aggregated microorganisms can be more readily cleared through mucociliary transport, intestinal peristalsis, and phagocytosis by immune cells.

4.5 Antiviral and Antifungal Activity

Defensins can bind viral envelopes, capsids, or host-cell surface molecules and thereby interfere with viral attachment, membrane fusion, and cellular entry.Their antifungal effects may involve plasma membrane damage, cell-wall stress, and disruption of intracellular ion homeostasis. The specific outcome depends on the defensin family member, whether the virus is enveloped, and the composition of the fungal cell wall.

Table 3. Major antimicrobial mechanisms of defensins

5 Defensin-Mediated Regulation of Immunity and Barrier Function

5.1 Immune-Cell Recruitment

Some β-defensins can promote the migration of immature dendritic cells, memory T cells, and monocytes toward sites of infection through chemokine receptor-associated mechanisms.Defensins function not only as antimicrobial effector molecules but also as signals of epithelial injury and infection, contributing to changes in the local immune-cell composition.

5.2 Regulation of Inflammatory Signaling

Defensins can bind microbial components such as lipopolysaccharides and alter the intensity of their stimulation of pattern-recognition receptors. They may also affect NF-κB, MAPK, and cytokine signaling in epithelial and immune cells.Controlled local defensin release facilitates pathogen clearance, whereas persistent overexpression or extensive extracellular release may amplify inflammation and tissue injury.

5.3 Epithelial Repair and Barrier Integrity

Molecules such as HBD-2 and HBD-3 can affect epithelial cell migration, proliferation, and cytokine release. Defensin expression during the early stages of infection helps restrict pathogen dissemination and supports barrier repair. Persistent abnormal expression may contribute to chronic inflammation, epithelial hyperplasia, and tissue remodeling.

5.4 Regulation of Microbial Community Structure

Defensins in the intestine, skin, and respiratory tract exert selective pressure on different microorganisms, influencing microbiota composition and the spatial relationship between microbes and epithelial surfaces.Abnormal Paneth cell function, granule release, or maturation of HD5 and HD6 may alter the intestinal microbiota and increase the likelihood that pathogens will reach the intestinal crypt region.

6 Defensins in Disease Research

6.1 Infectious Diseases

Bacterial, fungal, and viral infections can all alter defensin expression. Increased HNP levels are generally associated with neutrophil activation and degranulation, whereas elevated HBD-2 and HBD-3 more commonly reflect epithelial inflammatory responses.Increased defensin levels may represent effective host defense but may also accompany severe inflammation and tissue injury.

6.2 Inflammatory Bowel Disease

Abnormalities in Paneth cell function, granule release, autophagy, and endoplasmic reticulum homeostasis can all affect HD5 and HD6. Reduced Paneth cell α-defensins may occur in some forms of ileal Crohn’s disease, but this phenotype is also influenced by NOD2, ATG16L1, the severity of inflammation, and the intestinal microbiota. The disease cannot be attributed simply to the deficiency of a single defensin.

6.3 Skin and Respiratory Diseases

HBD-2 and HBD-3 can be markedly elevated in psoriasis, skin infections, and airway inflammation. The ionic strength, pH, protease activity, DNA content, and mucin content of respiratory mucus can all influence defensin activity. Local antimicrobial capacity may therefore remain insufficient even when defensin concentrations are normal or increased.

6.4 Urogenital Diseases

HBD-1, HBD-2, and other β-defensins participate in the basal defense of the urogenital epithelium. Infection, the hormonal environment, and the state of epithelial differentiation can all alter their expression.Tissue-specific members such as DEFB118 may also contribute to sperm-surface protection and microbial control in the reproductive tract.

6.5 Interpretation of Defensin Detection Results

ELISA primarily measures the concentration of immunoreactive defensins in a sample. It generally cannot directly distinguish precursor peptides from mature peptides, oxidized forms from reduced forms, or monomers from aggregates, nor can it demonstrate that the detected molecules retain antimicrobial activity.Salt concentration, pH, serum proteins, mucus components, and proteolytic degradation can all alter the actual biological effects of defensins.

Table 4. Major factors affecting the interpretation of defensin measurements

7 Products for Defensin Research

7.1 Products for Studying Human α-Defensins and HNPs

7.2 Products for Studying Human β-Defensins

7.3 Defensin Antibodies and Genetic Tools

7.4 Human Defensin Detection Kits

7.5 Mouse and Rat Defensin Detection Kits

Defensins establish a multilayered innate defense system through membrane activity, interference with intracellular processes, pathogen entrapment, and immune regulation. α-Defensins are distributed mainly in neutrophils and intestinal Paneth cells, whereas β-defensins primarily maintain epithelial barriers and respond to infectious stimuli. Defensin measurements should be interpreted according to the specific family member, maturation state, redox status, and local microenvironment.