Sebaceous glands are one part of the pilosebaceous unit, which also contains a hair follicle 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 called androgens. After about age 20, sebum production begins to decrease.
In the pilosebaceous unit, 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.
If the sebaceous gland does not produce enough sebum, the skin is dry. On the other hand, if it produces too much sebum the skin is oily, a condition called seborrhea. If sebum gets trapped in the pore, acne can develop.