Normal skin continually sloughs dead surface cells. When acne is present, proliferation of cells occurs at the neck and extends to the follicle. It's accompanied by excess sebum, causing the cells and bacteria to stick together. The combination of the excess sebum and dead skin cells leads to the formation of an impaction plug that creating the perfect environment for bacterial growth. The process of abnormal shedding of the dead skin cells results in a thickening of the outer layer of the skin.