The Skin's Defense Mechanisms: A Multi-Layered Barrier Against Infection

The skin is the body's primary physical and immunological barrier. Its defense operates through multiple integrated layers:

1. Physical Barrier

Stratum corneum: keratin-filled corneocytes in lipid matrix—impermeable to most microbes.

2. Chemical Barrier

• Acid mantle: pH 4.5–5.5 (lactic acid, urocanic acid, free fatty acids) inhibits pathogens.
• Antimicrobial peptides (AMPs): Defensins, cathelicidin (LL-37), RNase 7, dermcidin—direct microbial killing.
• Sebum: Lipids with antimicrobial properties; feeds commensals.

3. Microbiological Barrier

Resident microbiota (Staphylococcus epidermidis, Cutibacterium acnes, Corynebacterium) compete for nutrients, produce bacteriocins, and prime immune tone.

4. Cellular Immunity

• Langerhans cells: Epidermal dendritic cells; antigen capture → migration to lymph nodes → T-cell priming.
• Keratinocytes: Produce cytokines (IL-1, TNF-α), AMPs, and express TLRs.
• Dermal immune cells: Mast cells, macrophages, dermal dendritic cells, T cells (TRM).

5. Desquamation

Continuous shedding removes surface-bound microbes.

Disruption of any layer (trauma, antibiotics, immunosuppression, pH alteration) increases infection risk.