How Skin Protects You From Infection: A Physical Barrier

The skin is the body's largest organ and primary defense against pathogen entry. Its multi-layered barrier integrates physical, chemical, immunological, and microbial strategies.

1. Physical Barrier: The Epidermis

• Stratum corneum: 15-20 layers of anucleate corneocytes (keratin-filled) in lipid matrix (ceramides, cholesterol, free fatty acids) — "bricks and mortar."
• Tight junctions & desmosomes: Seal intercellular spaces in granular layer.
• Keratin: Insoluble fibrous protein — mechanical toughness, water resistance.
• Continuous desquamation: Sheds surface microbes with dead cells.

2. Chemical Barrier

• Acid mantle (pH 4.5-5.5): Lactic acid (sweat), urocanic acid, free fatty acids. Inhibits pathogens (S. aureus prefers pH 7), supports commensals.
• Sebum: Triglycerides → free fatty acids (antimicrobial); squalene (antioxidant); vitamin E. Occlusive, lubricates.
• Antimicrobial peptides (AMPs): Defensins (α, β), cathelicidin (LL-37), dermcidin (sweat), RNase7, psoriasin. Broad-spectrum: bacteria, fungi, viruses.
• Enzymes: Lysozyme (peptidoglycan cleavage), phospholipase A2.

3. Microbiological Barrier: Skin Microbiota

• Diverse communities: Cutibacterium, Staphylococcus, Corynebacterium, Malassezia (site-specific).
• Colonization resistance: Nutrient competition, niche exclusion, bacteriocin production, pH modulation.
• Immune education: Commensals train T-regs, Th17, IL-1/IL-17 axis.
• Dysbiosis → atopic dermatitis (S. aureus dominance), acne (C. acnes phylotypes), chronic wounds.

4. Immunological Barrier

• Resident immune cells: Langerhans cells (epidermal DCs), dermal DCs, macrophages, mast cells, γδ T cells, ILC2/3, tissue-resident memory T cells (TRM).
• Pattern recognition: TLRs, NLRs detect PAMPs/DAMPs → cytokine/chemokine release → neutrophil/monocyte recruitment.
• Humoral: IgA (secreted), IgG (transudated).

5. Barrier Disruption & Consequences

• Breaches: trauma, burns, ulcers, dermatitis, invasive devices.
• Pathogen entry → local infection (cellulitis, abscess) → systemic (sepsis).
• Atopic dermatitis: FLG mutation → barrier defect → allergen sensitization → "atopic march."

Maintenance

• Gentle cleansing (syndets, pH 5.5), moisturize (ceramides), UV protection, avoid irritants.
• Hand hygiene: alcohol-based rub (intact skin) or soap/water (soiled).