Skin Barrier Dysfunction

Skin barrier dysfunction is the defect in the outermost skin layer (the stratum corneum) that sits at the centre of Atopic Dermatitis. A healthy skin barrier works like a “brick wall”: dead skin cells (corneocytes) packed with keratin and Filaggrin-derived Natural Moisturising Factor, held together by a mortar of ceramides, cholesterol and free fatty acids. In eczema this wall leaks in both directions. Water escapes, measured as raised Transepidermal Water Loss (TEWL), so skin turns dry and itchy. Irritants, allergens and microbes get in. Ceramide levels are lower and their composition is shifted, and the lipids have shorter chains, which makes the skin more permeable (Agrawal & Woodfolk 2014). Barrier problems persist even in skin that looks clear, and how badly the barrier is impaired tracks disease severity (Elias & Wakefield 2011). That is the rationale for daily Emollient Therapy as baseline care.

Skin Barrier Dysfunction [defines] Atopic Dermatitis Skin Barrier Dysfunction [causes] Transepidermal Water Loss Filaggrin [part-of] skin barrier Emollient Therapy [treats] Skin Barrier Dysfunction

Genetics and family atopy. The best-established genetic cause is loss-of-function mutations in the Filaggrin gene (FLG). They are linked to early-onset, more persistent eczema and to later asthma (Agrawal & Woodfolk 2014). Carrying an FLG mutation raises eczema risk about 3.3-fold (Lee et al. 2024). How common these mutations are varies a lot by ancestry. In an Irish cohort, two mutations made up 80% of FLG mutations, compared with under 2% in a Singapore cohort (Agrawal & Woodfolk 2014). Other genes also matter. For example, SPINK5 mutations in Netherton Syndrome cause unchecked protease activity and severe eczema. In practice, family history is the strongest everyday risk marker. The NHS says a child is more likely to have atopic eczema if one or both parents have eczema, or a close relative has asthma or hay fever. Genes alone cannot explain the rapid rise in prevalence in industrialised countries, which points to environmental drivers (Lee et al. 2024).

Conflict: [Elias & Wakefield, Clin Rev Allergy Immunol, 2011] says up to 60% of Europeans with AD carry FLG mutations. [Furue, Int J Mol Sci, 2020] says rates range from 10% to 50% depending on ethnicity. Unresolved — add to open_questions.

Filaggrin [causes] Skin Barrier Dysfunction Netherton Syndrome [causes] Skin Barrier Dysfunction Atopic March [relates] Skin Barrier Dysfunction

Immune loop: the Th2 response and the skin microbiome. A leaky barrier lets antigens in, and the skin’s alarm signals (TSLP, IL-33, IL-1) push the immune system toward a Th2 Immune Response. The Th2 cytokines IL-4 and IL-13 then damage the barrier further. They suppress filaggrin, loricrin and involucrin through STAT6/STAT3 signalling, cut ceramide synthesis and reduce antimicrobial peptides (Furue 2020; Elias & Wakefield 2011). The result is a self-sustaining loop of itch, scratching and barrier damage. The microbiome adds to this. Eczema skin shows overgrowth of Staphylococcus aureus and lower bacterial diversity. In children during flares, S. aureus often made up more than 40% of the bacteria sampled (Edslev et al., Acta Derm Venereol 2020, citing Kong et al.). Low filaggrin and NMF raise skin pH, which favours S. aureus. Its toxins (alpha-toxin, superantigens) damage skin cells, drive IgE and worsen itch. Infected eczema, meaning weeping, crusted or pus-filled skin, needs urgent medical review because it can be bacterial infection or Eczema Herpeticum (NHS).

Th2 Immune Response [worsens] Skin Barrier Dysfunction Skin Barrier Dysfunction [triggers] Th2 Immune Response Staphylococcus aureus [worsens] Skin Barrier Dysfunction Skin Microbiome [relates] Skin Barrier Dysfunction

Environmental triggers. Many outside factors damage the barrier or provoke flares. These include soaps and detergents, heat and sweating, synthetic or wool fabrics, House Dust Mites, pet dander, pollen, low humidity and climate. The NHS lists soap, washing detergent, pets, some fabrics, pollen, house-dust mites, certain foods, heat, infections and stress. Alkaline soaps and surfactants such as Sodium Lauryl Sulfate raise skin pH and switch on protein-degrading enzymes (serine proteases). Low humidity speeds water loss, and stress hormones suppress barrier lipid synthesis (Elias & Wakefield 2011). Central heating dries the skin, while overheating directly triggers itch. High humidity encourages dust mites and mould. Cotton bedding is better tolerated than synthetics (National Eczema Society). For climate, US data on 79,667 children linked higher UV index and humidity with lower eczema rates, and children’s symptoms tend to worsen in autumn and winter (Lee et al. 2024). Hard Water is associated with childhood eczema (OR 1.28, 95% CI 1.09–1.50, very-low certainty), possibly because more detergent is left on the skin. However, two RCTs found water softeners did not improve established eczema (Jabbar-Lopez et al. 2021). Air pollution (PM2.5, NO2, VOCs), tobacco smoke and damp or mouldy homes are also associated with eczema (Lee et al. 2024). See Environmental Triggers.

Sodium Lauryl Sulfate [worsens] Skin Barrier Dysfunction Hard Water [relates] Skin Barrier Dysfunction House Dust Mites [triggers] Atopic Dermatitis Environmental Triggers [worsens] Skin Barrier Dysfunction

Food allergy: cause or association? And outside-in vs inside-out. Parents often suspect food. The current evidence mostly reverses the direction: eczema predisposes to Food Allergy more than food allergy causes eczema. Raised TEWL two days after birth predicted eczema at one year, and eczema usually comes before food allergy (Papapostolou et al., J Clin Med 2022). The Dual-Allergen Exposure Hypothesis explains why. Food proteins landing on inflamed skin (for example peanut in house dust, or peanut-oil skin products) cause sensitisation, while early eating builds tolerance. The LEAP Trial cut peanut allergy from 13.7% to 1.9% in high-risk infants with eczema who were fed peanut early. In a subset of children, foods can still trigger flares. But prolonged, unnecessary Elimination Diets can cause loss of tolerance, and even severe IgE reactions when the food is reintroduced. The same “which comes first” question shapes the two disease models. The inside-out model (historically dominant) says Th2 immune dysregulation comes first and damages the barrier. The outside-in model says the barrier defect is the primary driver that lets antigens in and starts inflammation (Elias & Wakefield 2011). The evidence is mixed. Barrier defects come before eczema, and IL-4/IL-13 can lower filaggrin by themselves, so acquired barrier loss is real. Yet filaggrin knock-down alone did not change stratum corneum permeability in a human skin model. Also, AD patients with and without FLG mutations showed similar penetration and Th2 responses after sensitisation through healthy skin (Agrawal & Woodfolk 2014). Most current reviews describe a two-way vicious cycle rather than a single starting point.

Conflict: [Elias & Wakefield, 2011] says the barrier defect is the primary “driver” of AD (outside-in) and barrier repair is more logical than immunosuppression. [Agrawal & Woodfolk, 2014] cites data that filaggrin loss alone does not raise SC permeability and that FLG status did not change sensitisation through healthy skin, favouring a convergent/immune-led view. Unresolved — add to open_questions.

Unsupervised food elimination diets

Cutting foods from a toddler’s diet without allergist confirmation risks nutritional gaps. It can also create IgE-mediated food allergy, with severe reactions on reintroduction (Papapostolou et al. 2022). Test and reintroduce under specialist supervision.

Skin Barrier Dysfunction [precedes] Food Allergy Dual-Allergen Exposure Hypothesis [defines] Food Allergy LEAP Trial [supports] Dual-Allergen Exposure Hypothesis Elimination Diets [causes] Food Allergy

Connections

• Atopic Dermatitis — barrier dysfunction is a core defining feature, source: Elias & Wakefield 2011
• Filaggrin — loss-of-function mutations are the main genetic barrier defect, source: Agrawal & Woodfolk 2014
• Th2 Immune Response — two-way vicious cycle with the barrier, source: Furue 2020
• Transepidermal Water Loss — the measurable output of a leaky barrier; predicts eczema at 1 year, source: Papapostolou 2022
• Natural Moisturising Factor — filaggrin breakdown product that keeps skin hydrated and acidic, source: Agrawal & Woodfolk 2014
• Staphylococcus aureus — colonises barrier-impaired skin and drives flares, source: Edslev 2020
• Skin Microbiome — reduced diversity in eczema, source: Edslev 2020
• Food Allergy — mostly a consequence of eczema via skin sensitisation, source: Papapostolou 2022
• Dual-Allergen Exposure Hypothesis — skin exposure sensitises, oral exposure tolerises, source: Papapostolou 2022
• LEAP Trial — early peanut feeding cut allergy by ~81% overall (86% in SPT-negative cohort: 13.7%→1.9%), source: Papapostolou 2022; Du Toit 2015
• Elimination Diets — risk of loss of tolerance, source: Papapostolou 2022
• Environmental Triggers — soaps, heat, fabrics, mites, pets, climate, source: NHS; National Eczema Society; Lee 2024
• Hard Water — weak association with childhood eczema; softeners no help, source: Jabbar-Lopez 2021
• House Dust Mites — common aeroallergen trigger, source: National Eczema Society
• Sodium Lauryl Sulfate — detergent that disrupts the barrier, source: Jabbar-Lopez 2021
• Netherton Syndrome — SPINK5 protease-inhibitor defect causing severe AD, source: Elias & Wakefield 2011
• Atopic March — barrier defect may underlie progression to asthma/rhinitis, source: Elias & Wakefield 2011
• Eczema Herpeticum — serious infection risk on barrier-impaired skin, source: NHS
• Emollient Therapy — first-line barrier repair, source: Elias & Wakefield 2011
• skin barrier — the healthy structure that fails here, source: Agrawal & Woodfolk 2014
• Skin pH — raised pH activates proteases and favours S. aureus, source: Elias & Wakefield 2011