What can go wrong with the barrier?
Skin disease is among the most common reasons people seek care — eczema and acne alone drive tens of millions of clinical visits, and skin cancer is the most common cancer of all. The categories organize cleanly by mechanism: barrier failures (eczema), immune accelerations (psoriasis — 28 days of turnover compressed into 3–7), the follicle's oil engine (acne), the cumulative UV ledger (photoaging and the three cancers), and the healing failures that read internal disease (diabetic and venous ulcers). And the field's unifying rule: the skin shows the inside — diabetes, thyroid disease, liver disease, and nutritional deficiency all write on the surface, often before they announce anywhere else.
The barrier failure: filaggrin genetics + immune overdrive → the itch-scratch spiral of cracked, inflamed skin. Management is moisturizer-first (the clinical instruction), prescription-tier second — topical steroids, calcineurin inhibitors, and now biologics for severe disease.
The immune acceleration: turnover compressed from 28 days to 3–7 — cells arriving at the surface before they're finished, stacking into scaly plaques. NOT contagious. Carries a comorbidity cluster (psoriatic arthritis, cardio-metabolic risk — the inside-out disease) and has been revolutionized by biologic therapy.
The pilosebaceous engine: oil + dead cells + C. acnes + inflammation, in the teenage near-universal and a persistent adult minority. Moderate-to-severe disease is prescription territory — retinoids, and isotretinoin for the severe end. The aisle borrows from a tier it isn't.
The cumulative UV ledger: wrinkles and spots on the cosmetic end; basal cell (the most common cancer in humans), squamous cell, and melanoma on the serious end. Prevention is SPF — the one intervention with an RCT (Chapter 5's Nambour story). Tanning beds are classified Group 1 carcinogens.
The flushing disorder: centrofacial redness with triggers (heat, alcohol, spice, sun), papules, and — importantly — an ocular form that deserves treatment. Prescription anti-inflammatory and antibiotic tiers; trigger mapping is the patient's side of the work.
The healing failures that read the vasculature: diabetic foot ulcers and venous stasis ulcers — the healing cascade stalled by glucose, pressure, and failing veins. Prevention is diabetes care (our diabetes page) and compression; treatment is wound-clinic territory.
Skin prevention has one giant lever, several honest habits, and a diagnostic skill: