Hair follicle
A hair follicle is the structure in the skin from which a hair grows. It extends from an opening at the surface down into the dermis, and it is usually paired with a sebaceous gland, the two together forming the pilosebaceous unit.
A follicle is best pictured as a narrow channel rather than a pit. It opens at the surface, passes through the epidermis and descends into the dermis, with the growing hair produced at its base.
Its depth matters more than its opening. Because a follicle reaches well below the surface, it provides a route into the skin that bypasses the barrier the outer layer otherwise presents, and several conditions exploit exactly that.
Attached to almost every follicle is a sebaceous gland, releasing sebum into the channel. The pair is treated as a single structure, the pilosebaceous unit, because what affects one affects the other. Acne is a condition of that unit rather than of the skin surface above it: the channel blocks, and inflammation follows inside it.
Small muscles attach to follicles too, and their contraction is what raises hairs when you are cold.
Follicles are not evenly distributed, and neither are the glands attached to them. Where they are densest is where certain conditions appear, which is why the face, scalp, chest and upper back recur so often in dermatology while the shins rarely do.
Some fungal infections involve the hair shaft itself, and scalp involvement is treated as its own category for that reason.
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