All acne is a disorder of the pilosebaceous unit, which is made 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.
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 it an oily appearance. When this process works correctly, the skin is moisturized and remains healthy.