Integumentary System

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Connected to · 3

synergy · 1

Immune
Skin is the immune system's front line: Langerhans cells stationed in the outer skin and the microbes living on its surface work together to keep invaders out. Atopic dermatitis is what happens when two things fail at once: the skin barrier is damaged, and the immune response is skewed toward type 2 inflammation (Th2).

regulates · 2

Bone
Skin is where vitamin D starts: UVB in sunlight converts 7-dehydrocholesterol in the skin into a precursor of vitamin D, which the liver and kidney then process into the active form that helps the gut absorb calcium. Skin tone, sunscreen and latitude all affect bone at this first step in the skin.
Endocrine
Skin is not only where vitamin D is made; it can also make and convert cortisol and sex hormones locally. Some of the eczema, hair loss and acne that appear under long-term stress may involve the skin's local stress-hormone pathway and the enzyme aromatase; these mechanisms come mainly from laboratory research.

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Three layers and how fast they renew

EVIDENCEGUIDE 4

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The outermost layer of your skin is replaced every few weeks, but the collagen deep underneath takes years to renew. So whatever you see change within a few weeks of a new diet or skin routine is mostly the surface.

Skin has three layers. The epidermis has no blood vessels. Its main cells, keratinocytes, are born in the deepest basal layer and take about 28 days to reach the surface, die and flake off. The dermis is the layer that holds the skin up, woven from collagen, elastin and water-holding hyaluronic acid, and these components have half-lives measured in years; once a lot of collagen is lost, it rarely grows back. Below that, the hypodermis is a pad of fat. Besides keeping out microbes and holding in water, skin senses heat, cold, touch and pain, controls body temperature through sweat and blood vessels, makes vitamin D from ultraviolet light, and hosts resident immune cells.

Skin also has emergencies that cannot wait. If a medicine is followed by widespread red rash, blisters, sores on the lining of the mouth or eyes, and fever, it may be a severe drug reaction (Stevens-Johnson syndrome or toxic epidermal necrolysis), and you need medical care immediately.

Educational content, not medical advice — consult a clinician.

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