Acne is a disorder of the pilosebaceous units in the skin. These are found most abundantly in the skin over the face, the upper chest, and the back. They are responsible for the growth of skin hair in addition to the lubrication of the skin.
The pilosebaceous units consist of a fine canal lined by keratinocytes, called the hair follicle, and an attached secretory gland called a sebaceous gland. The keratinocytes give rise to an outgrowth composed of keratin, a fibrous waterproof protein. This makes up a hair shaft, which emerges towards the skin by a tiny orifice, popularly called a skin pore.
The sebaceous gland produces a thick oily secretion called sebum which is secreted into the hair follicle. The sebum slowly reaches the skin surface along with dead cells from the keratinocyte layer, and other debris. If the canal of the hair follicle is plugged, a comedo results. This is the primary lesion of acne. It consists of a bump formed by accumulated sebum and the dead skin cells dilating a blocked follicle.