Integumentary System: Skin's Protective Functions

The integumentary system (skin, hair, nails, glands) is the body's largest organ and first line of defense.

Protective Roles

Threat	Skin Defense Mechanism
Mechanical injury	Stratified squamous epithelium (keratinized) — tough, flexible barrier; subcutaneous fat cushions.
Pathogens	Acid mantle (pH 4.5–5.5), antimicrobial peptides (defensins, cathelicidins), sebaceous lipids, resident microbiome competition, Langerhans cells (immune surveillance).
Dehydration	Stratum corneum lipids (ceramides, cholesterol, fatty acids) limit transepidermal water loss (TEWL).
Thermal dysregulation	Vasoconstriction/vasodilation, sweating, piloerection, subcutaneous insulation.
UV radiation	Melanin (eumelanin) absorbs/scatters UV; stratum corneum thickness; DNA repair enzymes.
Chemicals	Keratin barrier limits penetration; metabolic enzymes (CYPs, esterases) detoxify some xenobiotics.
Allergens	Barrier limits penetration; Langerhans cells present antigen → tolerance or immune response.

Additional Functions

• Vitamin D₃ synthesis (UVB → 7‑dehydrocholesterol).
• Sensory reception (touch, pressure, temperature, pain).
• Excretion (sweat: water, electrolytes, urea).
• Psychosocial: appearance, identity, communication.