The disorder of pilosebaceous units consisting of a hair, hair follicle and sebaceous gland causes acne. The pilosebaceous units are present all over the body, apart from the front part of our feet, lower lips, soles and palms
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The sebaceous gland releases sebum which helps to keep the hair and skin moist. With age the production of sebum tends to reduce. Acnes are caused by a particular kind of bacteria known as Propionibacterium which is a common resident of the skin. It treats sebum as nourishment for its growth. Individuals suffering from acne have more of these bacteria in their follicles in comparison to those without acne. The existence of these bacteria draws more white blood cells to the hair follicles. These blood cells produce a particular enzyme which causes harm to the follicle wall permitting the follicle contents to enter the dermis. This process results in the formation of acne on the skin layer.

The Propionibacterium also helps in the development of free fatty acids which causes the inflammatory procedure to increase in the follicle.  When sebum gets trapped in hair follicles, it results in blockage of the follicle because of which the cells that are usually sloughed off tend to become sticky. This causes the formation of whiteheads and blackheads. This process is known as comedogenesis.