Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is almost completely covered with hair follicles. The part of a hair that is within the follicle is the hair root. Human hair is a protein filament, primarily composed of keratin, that grows from follicles in the skin and serves various functions, including insulation, protection, and acting as a sense organ.