[noun] Sebum is an oily substance secreted by the sebaceous glands. It is composed mainly of fatty acids and plays a protective role as a component of the hydrolipidic film. In association with sweat, it preserves the hydration and suppleness of the epidermis. By virtue of its fatty acids, it forms a shield against microbes. Increased sebum secretion means its natural evacuation is hampered, which can lead to obstructed pores. The skin may then become shiny and blackheads may form.