Acne is believed to result from a change in the inner lining of the follicle that prevents the sebum from passing through. For reasons not understood, cells from the lining of the follicle are shed too fast and clump together. The clumped cells plug up the follicle’s opening so sebum cannot reach the surface of the skin. The mixture of oil and cells causes bacteria that normally live on the skin, called Propionibacterium acnes (P. acnes), to grow in the plugged follicles. These bacteria produce chemicals and enzymes that can cause inflammation. (Inflammation is a characteristic reaction of tissues to disease or injury and is marked by four signs: swelling, redness, heat, and pain.) When the plugged follicle can no longer hold its contents, it bursts and spills everything onto the nearby skin—sebum, shed skin cells, and bacteria. Lesions or pimples develop as a result of the skin’s being irritated.