Hair refers to naturally growing, thread-like projections made from bundles of keratin – a protein also found in teeth and nails (which is why hair removal products can damage all three). Although organisms like insects and spiders sport similar filamentous structures, hair is unique to mammals.
Hair is produced by follicles, narrow cavities found in the skin. Mammals express hair on all non-glabrous skin – the glabrous skin on lips, eyelids, palms, soles, back of the ears, and parts of the genitals lack follicles. Each follicle’s sebaceous glands produce oily sebum that lubricates and waterproofs the skin and hair. A hair matrix at the follicle's base generates hair through rapid cell division; cancer treatments such as chemotherapy specifically target rapidly dividing cells, causing hair loss side effects.