What Is Acne Vulgaris?
Acne vulgaris is a chronic skin condition affecting the pilosebaceous unit, the structure formed by a hair follicle and its attached sebaceous gland. It develops when a follicle becomes blocked by keratin and sebum and inflammatory processes begin within and around the follicle.
Acne most commonly affects areas with many sebaceous glands, particularly the face, upper back, chest, and shoulders. It can produce several different types of acne lesions, including blackheads, whiteheads, papules, pustules, nodules, and deeper cyst-like lesions.
Although acne is often associated with adolescence, it can occur at different ages. The condition reflects the interaction of several biological processes, including sebum production, follicular plugging, microbial activity, hormonal influences, and inflammation.
The Skin Structures Involved in Acne
The main structures involved in acne are the pilosebaceous units. Each unit consists of a hair follicle, hair shaft, sebaceous gland, and the canal through which material from the follicle reaches the skin surface.
The sebaceous gland produces sebum, an oily mixture of lipids that helps lubricate and protect the skin and hair. The inside of the follicle is lined with keratinocytes, the same general type of cell found in the outer layer of the skin.
Under normal conditions, keratinocytes are shed in an orderly way and leave the follicle through its opening. In acne, these cells can accumulate and combine with sebum, making the follicle more likely to become blocked.
The immune and endocrine systems also influence the pilosebaceous unit. Hormonal signals can affect sebaceous gland activity, while immune responses contribute to the inflammation that produces red, swollen, or painful acne lesions.
How Does Acne Develop?
Acne usually begins with a microscopic blockage inside a hair follicle. Abnormal accumulation of keratinocytes and sebum creates a plug known as a microcomedo. A microcomedo is the precursor from which visible acne lesions can develop.
As sebum and keratin continue to accumulate, the blocked follicle may develop into a comedo. If the follicular opening remains open, the lesion can become an open comedo, commonly known as a blackhead. If the opening remains closed, it can become a closed comedo, commonly known as a whitehead.
The blocked follicle also creates an environment in which Cutibacterium acnes, a bacterium normally present on human skin, can become more biologically active. Changes within the follicle can stimulate the immune system and increase local inflammation.
If inflammation intensifies, the follicular wall can become damaged. Material from inside the follicle may then enter the surrounding skin, producing a stronger inflammatory response. This can result in papules, pustules, nodules, or deeper inflammatory lesions.
Acne therefore develops through several interacting processes rather than one single cause. Follicular blockage, sebum production, microbial activity, and inflammation all contribute to the development and progression of acne vulgaris.
Types of Acne
There are several types of acne lesions. They differ according to whether the follicle is primarily blocked, inflamed, or affected more deeply within the skin.
Comedonal acne
Comedonal acne is dominated by blocked follicles known as comedones. These include:
- Open comedones (blackheads) – follicles in which the opening remains exposed to the air.
- Closed comedones (whiteheads) – blocked follicles in which the opening remains covered by skin.
Blackheads are not dark because they contain dirt. The exposed material within an open comedo darkens through chemical changes when it comes into contact with air.
Inflammatory acne
Inflammatory acne develops when immune activity increases within and around a blocked follicle. Common inflammatory lesions include:
- Papules – small, raised, inflamed lesions without a visible collection of pus.
- Pustules – inflamed lesions containing a visible collection of inflammatory material.
Nodular and deeper acne
When inflammation extends deeper into the skin, larger and more painful lesions can develop. Nodules and cyst-like acne lesions involve deeper tissue than ordinary papules or pustules and have a greater potential to damage surrounding skin structures.
People can have more than one type of acne lesion at the same time. For example, blackheads, whiteheads, papules, and pustules may all occur within the same affected area.
Where Does Acne Occur?
Acne most often develops in areas containing large numbers of active sebaceous glands. These commonly include:
- the face
- the forehead and jawline
- the chest
- the shoulders
- the upper back
The distribution varies between individuals. Some people mainly develop facial acne, while others experience significant acne on the back, chest, or several areas of the body.
What Causes the Follicle to Become Blocked?
Follicular blockage is associated with abnormal keratinization, in which cells lining the follicle accumulate instead of being shed normally. These cells combine with sebum and form material that obstructs the follicular canal.
Sebum production, hormonal activity, genetic influences, microbial activity, inflammatory responses, and environmental factors can all influence the likelihood that acne will develop or persist.
Because these factors are complex and can differ between individuals, they are covered in more detail in the Causes of Acne vulgaris guide.
The Role of Sebum and Sebaceous Glands
Sebaceous glands are closely involved in acne because they produce the sebum that enters the hair follicle. Sebum normally contributes to the lubrication and protection of the skin, but increased production or changes within the follicle can contribute to the environment in which acne develops.
Sebaceous gland activity is strongly influenced by androgen hormones. This helps explain why acne commonly becomes more noticeable around puberty, when androgen activity increases and sebaceous glands become more active.
However, acne is not simply caused by oily skin. Sebum interacts with follicular keratinization, microbial activity, and immune responses, so several biological processes must be considered together.
The Role of Cutibacterium acnes
Cutibacterium acnes is part of the normal population of microorganisms living on human skin. Its presence does not mean that acne is simply a bacterial infection.
Within a blocked follicle, however, conditions can favour changes in bacterial activity. Bacterial products can interact with skin cells and immune pathways and contribute to inflammation.
For this reason, acne is better understood as an inflammatory disorder involving the follicle and its microbial environment rather than as a conventional skin infection.
How Acne Changes the Skin
Early acne may involve only microscopic follicular blockage. As the condition progresses, visible comedones or inflammatory lesions can develop.
Inflammation causes local blood vessels to dilate and immune cells to enter the affected tissue. This produces the redness, swelling, tenderness, and raised appearance associated with inflammatory acne.
If inflammation becomes deep or repeatedly damages the surrounding dermis, normal collagen architecture can be disrupted. Healing may then leave changes in skin texture or pigmentation.
How Acne Can Affect the Skin Over Time
Acne can involve repeated cycles of follicular blockage, inflammation, healing, and formation of new lesions. Some lesions resolve without leaving a visible change, while others may leave temporary colour changes or permanent alterations to the skin.
Deep or persistent inflammatory acne has a greater risk of producing acne scars because inflammation can damage collagen and other supporting structures within the dermis.
The skin may therefore contain acne lesions at several stages at the same time, including newly blocked follicles, active inflammatory lesions, healing lesions, pigmentation changes, and older scars.
Is Acne a Chronic Condition?
Acne vulgaris is considered a chronic inflammatory disorder because the biological processes responsible for acne can continue over an extended period. Individual lesions may appear and disappear, but the tendency for follicles to become blocked and inflamed may persist.
The severity and duration of acne vary considerably between individuals. Some people experience relatively mild comedonal acne, while others develop persistent inflammatory or nodular acne affecting larger areas of skin.
Summary
Acne vulgaris is a chronic disorder of the pilosebaceous unit. It develops when hair follicles become blocked by keratin and sebum and biological changes within the follicle promote microbial activity and inflammation.
The main types of acne include comedonal acne, involving blackheads and whiteheads; inflammatory acne, involving papules and pustules; and deeper forms involving nodules and cyst-like lesions. Acne commonly affects the face, chest, shoulders, and back because these areas contain many active sebaceous glands.
Understanding how acne forms also explains why different acne lesions can occur together and why persistent or deep inflammation can sometimes lead to pigmentation changes or scarring.