All acne is a disorder of the pilosebaceous unit, which is made up of a hair follicle, sebaceous gland, and a hair. These units are found everywhere on the body except on the palms, soles, top of the feet, and the lower lip. The number of pilosebaceous units is greatest on the face, upper neck, and chest. Sebaceous glands produce a substance called sebum, which is responsible for keeping the skin and hair moisturized. During adolescence sebaceous glands enlarge and produce more sebum under the influence of hormones, also called androgens. After about age 20, sebum production begins to decrease.

Acne & Bacteria
A bacteria, known as Propionibacterium acnes, is a normal inhabitant of the skin. It uses sebum as a nutrient for growth, therefore increases in follicles during puberty. People with acne have more Propionibacterium acnes in their follicles than people without acne. The presence of bacteria attracts white blood cells to the follicle. These white blood cells produce an enzyme that damages the wall of the follicle, allowing the contents of the follicle to enter the dermis. This process causes an inflammatory response seen as papules (red bumps), pustules, and nodules. The bacteria also cause the formation of free fatty acids, which are irritants, increasing the inflammatory process in the follicle.

Normal Follicles
Sebum produced by the sebaceous gland combines with cells being sloughed off within the hair follicle and "fills up" the hair follicle. When the follicle is "full", the sebum spreads over the skin surface giving the skin an oily appearance. When this process works correctly, the skin is moisturized and remains healthy.

Obstructed Follicles
Problems arise when the sebum is trapped in the hair follicle. For reasons that are still unclear, some hair follicles become obstructed. The sebum is produced but gets trapped on the way out, and the cells that are normally sloughed off become "sticky", plugging up the follicle. The process of obstructing follicles is called comedogenesis. It causes some follicles to form a type of acne called comedones, also known as blackheads and whiteheads.

For most people with acne, it begins at puberty. This is when the body starts to produce hormones called androgens. Androgens cause the enlargement and overstimulation of the sebaceous glands in people with acne. Sensitivity to these androgens also cause acne to appear during the menstrual cycle and sometimes in pregnancy. The extra sebum (oil) that the sebaceous glands produce mixes with dead skin cells and bacteria on the skin's surface and this blocks pores. Inside the blocked pore, bacteria multiply and cause inflammation leading to the spots and pustules associated with acne.