The sebaceous glands make an oily substance called sebum that normally empties onto the skin surface through the opening of the follicle, commonly called a pore. Cells called keratinocytes line the follicle. The sebaceous glands which lie just under the surface of your skin produce sebum, the skins natural moisturizer. These glands and the hair follicles where they are found are called sebaceous follicles. This trapped sweat, mixed with the oil secreted by the sebaceous glands, causes the skin pores on the back to clog and the result is acne.