Illustration of a pimple. A sebum plug (clear) has blocked the pore from a hair follicle at the surface of the skin. Sebum, produced by sebaceous glands within the skin, is an oily substance that lubricates and protects the skin. Overproduction of sebum can lead to blocked pores, which allow bacteria (pink rods) present to multiply. White blood cells (white), part of the body's immune system, infiltrate the blockage and cause inflammation and the formation a pimple or pustule.