Shaving gives the tip of the hair a sharp point, which can push against the wall of the hair follicle or the skin, blocking the exit of the hair follicle. As a result, the hair can grow under the top layer of skin. A sebaceous gland uses this same outlet as the hair, in order to keep the hair supple during normal growth. The sebaceous gland comes under pressure due to the blocked outlet, which can lead to inflammation with pus, swelling and redness.