Acne lesions originate in the hair follicle and sebaceous gland which is called the pilosebaceous unit (PSU). The PSU is made of 3 components.

1. Sebaceous gland– This gland produces a substance called sebum. Sebum is an oily product that is made up of different lipids. Sebum coats the hair and skin to keep your skin lubricated and protected. It also assists in taking the dead skin cells to the exterior of the skin. There can be an excessive production of sebum in acne prone individuals.
2. Sebaceous duct– This duct connects the sebaceous gland to the exterior skin.
3. Hair shaft– These are the actual strands of hair found on our skin.

There are 4 main factors that play a role in the development of acne at the level of the pilosebaceous unit.

1. Abnormal keratinization– The dead skin cells and keratin should be sloughed off at the skin surface. If they do not slough off, then the dead skin cells and the keratin form a plug and clog up the hair follicle.
2. Sebum production– Certain hormones called androgens are normally produced in the body, they are mainly male hormones but also exist in women in smaller amounts. The most common androgen is testosterone. Androgens increase in adolescence and cause the sebaceous glands to become larger and produce more sebum. As the teenage years end, the androgen levels reduce and acne tends to improve. However, some continue to have increased sensitivity to androgens resulting in acne in adulthood.
3. Bacteria– The main bacteria in acne is Propionibacterium acnes. It is actually normal to have this bacteria in the skin. However, the levels of P. acnes can go up because of increased sebum production. P. acnes produces lipase which converts the lipids in the sebum to free fatty acids. FFA act as an irritant to the skin contributing to acne lesions.
4. Inflammation– The body then reacts to the bacteria and FFA by sending white blood cells (neutrophils) to kill the bacteria. It also sends other white blood cells (lymphocytes and macrophages) to repair the tissue. Everybody’s inflammatory response is different. Some are more prone to inflammation especially teenagers. Pictures describing the pathogenesis

Types of Acne lesions

Comedones– These are noninflammatory lesions called whiteheads and blackheads. They develop when the hair follicle is blocked by a keratin plug trapping the sebum underneath. As the trapped sebum increases, it creates pressure which ends up becoming a comedone. There are 2 types:

• Closed comedone– a whitehead, a noninflamed lesion with a white center that stays below the surface of the skin
• Open comedone– Blackhead, noninflammatory lesion with a dark centre. It develops when the comedone pops out through the surface of the skin, exposing the melanin ( dark pigment in the skin) to oxygen in the air.

Papules– inflammatory lesions that are small firm and red commonly referred to as pimples.

Pustules– a papule that contains pus, whitish substance which contains a lot of white blood cells. It usually is a red pimple with a white center.

Cyst / Nodule– Tender pus filled papule or pustule that is deeper in the skin. They occur when the immune system tries to contain the invader (bacteria) and the epidermis wraps around the hair follicle. When the pus hardens inside, the lesion becomes a nodule.