A comedo is a plugged hair follicle in the skin, the foundational lesion behind nearly every case of acne. The plug forms when dead skin cells and oily sebum accumulate inside the follicle instead of shedding normally. When the follicle stays closed at the surface, the trapped material creates a small, flesh-colored or whitish bump commonly called a whitehead. When the opening stretches wide enough for the plug to contact air, oxidation darkens it into the familiar blackhead. Both types can sit quietly for months, progress into inflamed pimples, or resolve on their own, and the biology driving their formation is more layered than most people realize.
How a Comedo Forms
Skin cells lining the inside of a hair follicle normally mature, loosen, and get swept out to the surface along with sebum. In comedo-prone skin, that process goes sideways. The cells stick together rather than sloughing off, a phenomenon called follicular hyperkeratinization. This sticky lining narrows the exit and traps sebum behind it, gradually inflating the follicle into a microcomedo too small to see with the naked eye. Over weeks, the microcomedo can enlarge into a visible open or closed comedo.
What triggers the sticky lining? Several forces converge. One is the skin’s own inflammatory signaling. The bacterium Cutibacterium acnes (formerly Propionibacterium acnes) colonizes follicles and prompts surrounding cells to release interleukin-1 alpha, a signaling molecule that accelerates the buildup of keratin in the follicle wall.1Journal of Biomedical and Allied Research. The Obsidian Impediment Comedones This means the earliest stage of a comedo already involves a conversation between microbes and the immune system, not just a passive clogging of pores.
The Chemistry Inside the Plug
Sebum is not a single substance. It is a complex mixture of fats produced by sebaceous glands attached to each hair follicle. Among those fats, squalene stands out. It makes up roughly 12% of sebum and is unique to the skin’s oil glands; internal organs do not accumulate it. The reason squalene piles up in sebum rather than being converted into cholesterol, as it would be elsewhere in the body, likely comes down to environment: the sebaceous gland is low in oxygen, and the enzyme that processes squalene into the next step toward cholesterol needs molecular oxygen to work.2Journal of Lipid Research. Sebaceous glands
In people with acne, squalene levels jump. One study comparing acne-prone skin with clear skin found squalene was elevated roughly twofold in acne subjects and represented about 20% of total sebaceous lipids, compared with 15% in controls.3PubMed Central. Sebum analysis of individuals with and without acne That matters because squalene is vulnerable to a specific kind of damage on the skin surface: ultraviolet light triggers singlet oxygen oxidation, converting squalene into squalene hydroperoxide, a compound linked to irritation and further follicular disruption.4PubMed. Linoleic acid and squalene are oxidized by discrete oxidation mechanisms in human sebum In other words, higher squalene does not just reflect oilier skin; it creates a chemical environment more prone to the oxidative damage that can worsen comedone formation.
Why Hormones and Diet Both Matter
Sebaceous glands respond strongly to androgens. Testosterone and its more potent derivative, dihydrotestosterone, bind to receptors inside the oil-producing cells and crank up sebum output. This hormonal sensitivity explains why comedones and acne tend to erupt at puberty, flare around menstrual cycles, and sometimes improve with anti-androgen medications. Androgen-driven excess sebum production is considered one of the earliest steps in acne development and remains a primary therapeutic target.5PubMed. The cutaneous effects of androgens and androgen-mediated sebum production and their pathophysiologic and therapeutic importance in acne vulgaris
Hormones from outside the skin also play a role. Insulin and insulin-like growth factor 1 (IGF-1) amplify sebaceous gland activity through intracellular growth-signaling pathways.6PubMed. Endocrine and metabolic drivers of sebum dysregulation: Mechanisms and therapeutic strategies This is where diet enters the picture. Foods that spike insulin, particularly high-glycemic carbohydrates and concentrated dairy, push the same metabolic switches that androgens do. During puberty, when IGF-1 is already elevated, adding a high-glycemic diet on top may amplify sebum production and the expression of fat-building enzymes in the gland, promoting both the quantity and composition of sebum that favors comedo formation.7PubMed Central. Linking diet to acne metabolomics, inflammation, and comedogenesis: an update
The practical takeaway is that comedones are not purely a hygiene issue. You can wash your face meticulously and still develop them if your hormonal and metabolic environment is driving excess sebum and abnormal follicular lining. This is why treatments that only address surface oil rarely solve the problem on their own.
Microbes and Biofilms
Cutibacterium acnes lives deep in follicles and is a normal part of skin flora. It becomes problematic in part through biofilm formation, a mode of growth where bacteria embed themselves in a protective matrix that antibiotics and the immune system have difficulty penetrating. Research on mild-to-moderate acne found that biofilms were detected in more than half of comedone samples, compared with only about a fifth of inflammatory papules and uninvolved skin.8PubMed. Noninflammatory comedones have greater diversity in microbiome and are more prone to biofilm formation than inflammatory lesions of acne vulgaris The same study found that comedones harbored a more diverse bacterial community than inflamed lesions. This is somewhat counterintuitive: the “quieter” lesion turns out to be more microbiologically complex and more biofilm-prone.9PubMed Central. Recent advances in understanding Propionibacterium acnes (Cutibacterium acnes) in acne
Biofilm presence in comedones may help explain why some blackheads and whiteheads stubbornly resist topical treatments. If bacteria inside the plugged follicle are shielded by a biofilm matrix, simply applying an antimicrobial to the skin surface may not reach them effectively. This is an active area of research, with some dermatologists exploring combination approaches specifically aimed at disrupting biofilms.
Open Versus Closed Comedones in Practice
Though both types share the same underlying mechanism, they behave differently on the skin and respond differently to treatment. A closed comedo (whitehead) has a very narrow or sealed follicular opening, so the plug never contacts air and stays pale. These are the ones more likely to become inflamed, because the sealed environment builds pressure and concentrates bacterial byproducts. An open comedo (blackhead) has a wider opening. The dark color is not dirt; it comes from oxidation of melanin and lipids at the surface of the plug.
Extraction, the manual removal of a comedo’s contents using a small loop tool, is generally more straightforward with open comedones because the pore is already dilated. Closed comedones sometimes need a tiny nick in the overlying skin to release the plug, which is why dermatologists discourage aggressive at-home squeezing. Forcing a closed comedo can rupture the follicle wall internally, spreading its contents into surrounding tissue and triggering a deeper inflammatory response than the comedo would have caused on its own.
Topical Treatments That Target Comedones
Retinoids are the gold standard for comedonal acne. Topical tretinoin, a derivative of vitamin A, works by normalizing the way follicular lining cells mature and shed, essentially correcting the sticky-cell problem at the root of comedo formation. It also promotes turnover of the outer skin layer and reduces the buildup of keratinized material inside the follicle.10PubMed Central. An Updated Review of Topical Tretinoin in Dermatology: From Acne and Photoaging to Skin Cancer Adapalene and tazarotene are related retinoids with similar comedolytic activity and varying tolerability profiles.
Salicylic acid takes a different approach. Because it is lipophilic (oil-soluble), it can penetrate through the oily sebum into the follicle itself, where it dissolves the keratin plug from the inside out. This gives it an advantage over water-soluble exfoliants at reaching the site where comedones actually form.11Journal of Cutaneous and Aesthetic Surgery. Theoretical and potential combined use of salicylic acid, azelaic acid, and lactic acid in acne care Many over-the-counter acne cleansers and leave-on products use salicylic acid at concentrations between 0.5% and 2%, which is enough to help with mild comedonal acne but may not be sufficient for dense, stubborn whiteheads.
Benzoyl peroxide, while primarily an antimicrobial, also has mild comedolytic effects and is often combined with a retinoid. The combination targets both the keratin plug and the bacterial biofilm inside it, addressing two of the major forces sustaining comedones simultaneously.
Comedones from Friction, Chemicals, and Sun Damage
Not all comedones stem from the classic acne pathway. Acne mechanica occurs when repeated friction, pressure, or occlusion triggers comedone formation in areas that do not usually break out. Helmet straps, tight clothing, backpack straps, and even prolonged pressure against a chair can cause it. One published case documented bilateral open comedones on the inner thighs caused purely by skin-on-skin friction during walking.12PubMed. Inner thigh friction as a cause of acne mechanica The comedones look identical to ordinary blackheads but resolve once the mechanical irritant is removed.
Chloracne is a far more serious variant. It arises from systemic poisoning by halogenated aromatic hydrocarbons, with dioxin being the most potent trigger. The comedones in chloracne tend to be widespread, persistent, and resistant to standard acne treatments because the underlying cause is toxic exposure rather than hormonal or follicular dysfunction.13PubMed Central. Environmental pollution and acne: Chloracne Chloracne is considered one of the most reliable clinical markers of dioxin exposure and has appeared in industrial accidents and cases of deliberate poisoning.
Chronic sun damage produces its own distinctive pattern. Favre-Racouchot disease presents as clusters of large open comedones and small cysts on sun-weathered skin, typically around the eyes and temples of older, fair-skinned individuals. Decades of ultraviolet exposure degrades the elastic tissue in the dermis, distorting follicle architecture and allowing comedones to form in skin that may never have been acne-prone in youth. Smoking and prior radiotherapy can contribute as well.14PubMed. Favre-Racouchot disease: protective effect of solar elastosis15PubMed. Favre-Racouchot disease: systematic review and possible therapeutic strategies The condition is benign but cosmetically bothersome, and the comedones often respond well to gentle extraction or topical retinoids.
When It Looks Like a Comedo but Isn’t
Several conditions mimic comedones closely enough to cause confusion, even among clinicians at first glance.
- Milia: Tiny white keratin cysts, usually under 3 mm, that sit just beneath the surface. They look like closed comedones but are actually miniature cysts with a fully formed wall, and they do not respond to comedolytic treatments the way whiteheads do. They are typically removed with a small needle or left alone.
- Sebaceous hyperplasia: Yellowish, slightly dome-shaped bumps caused by benign overgrowth of oil glands. They can resemble whiteheads, but if you try to extract one, you get an oily secretion rather than a solid plug.
- Steatocystoma: Persistent cystic lesions that mimic closed comedones and do not respond to typical acne treatment. Suppurating versions can resemble severe cystic acne.
A dilated pore of Winer is another mimic worth knowing about. It presents as a single, prominent open pore, often on the face, filled with laminated keratin. Histologically, it differs from a simple large comedone because the follicular invagination extends deep into the dermis and has characteristic finger-like projections of tissue pushing into the surrounding skin.16Indian Journal of Postgraduate Dermatology. Dilated Pore of Winer; What’s That! To the untrained eye, it just looks like a stubborn blackhead that keeps refilling, but it is a benign growth with its own distinct structure.
Comedogenicity Testing and Cosmetic Products
The term “comedogenic” on a product label traces back to a testing method developed in the late 1970s. Researchers applied substances to rabbit ears and then examined the tissue under magnification for signs of follicular plugging. Materials were ranked on a comedogenicity scale based on how much hyperkeratosis they produced.17PubMed. An improved rabbit ear model for assessing comedogenic substances This rabbit ear assay became the foundation for “non-comedogenic” marketing claims on skincare and makeup products.
The model has real limitations. Rabbit ears are more sensitive than human skin, so a substance that scores high in the assay may not actually cause comedones in people. Concentration matters too: an ingredient comedogenic at 100% purity may be harmless at the 1-2% concentration used in a finished product. Human patch testing and repeat-insult studies have been added to refine the picture, but no single test perfectly predicts whether a product will clog your pores. If you are comedo-prone, “non-comedogenic” on the label is a reasonable starting filter, not a guarantee.
Genetics of Comedo-Prone Skin
Acne runs in families, and genome-wide association studies have started to quantify the genetic component. A large meta-analysis identified 29 novel genetic loci associated with acne risk and estimated that common genetic variants across the genome explain roughly 23% of the variation in acne susceptibility.18Nature Communications. Genome-wide association meta-analysis identifies 29 new acne susceptibility loci A subsequent study, pooling data from over 34,000 acne cases and nearly 365,000 controls, brought the total number of identified risk loci to 50 and found that people in the top 5% of genetic risk had about 1.6 times the acne risk compared to average.19European Journal of Human Genetics. Genome-wide meta-analysis identifies novel loci conferring risk of acne vulgaris
Some of the implicated genes relate to fatty acid synthesis, follicular structure, and androgen metabolism, which maps neatly onto the biological pathways already discussed. But no single gene “causes” comedones. The genetic picture is one of many small-effect variants, each nudging sebum composition, follicular keratinization, or immune responsiveness slightly in the wrong direction. This polygenic architecture explains why acne severity varies so widely even among siblings.
Comedones That Cross Species
Comedones are not exclusive to humans. Feline chin acne is a well-recognized condition in cats, presenting as blackheads and crusts on the chin and lower lip. The underlying pathology involves dilated sebaceous gland ducts and, when infected, inflammatory infiltration similar to what happens in human acne.20PubMed Central / Wiley Online Library. Feline acne and results of treatment with mupirocin in an open clinical trial: 25 cases (1994-96) Cat owners sometimes notice dark specks on their pet’s chin that look like flea dirt but are actually comedonal plugs. Mild cases are managed with gentle cleansing, while severe or infected cases may require topical or systemic antimicrobials.
The Skin-Picking Complication
For some people, the presence of comedones becomes an obsessive focus. Acne excoriée is a recognized skin-picking disorder within the obsessive-compulsive spectrum, characterized by compulsive manipulation of acne lesions far beyond what any reasonable extraction would involve. It appears most often in adolescent or young adult women and can produce scarring and disfigurement that far exceeds what the original comedones would have caused. Treatment usually involves both dermatological care for the skin damage and psychological support, sometimes including cognitive behavioral therapy or medication for the compulsive behavior itself.21PubMed Central / Wiley Online Library. Acne excoriée: Diagnostic overview and management The lesson here is that the urge to pick at comedones, while almost universal, can cross into a clinical territory where the “treatment” causes more harm than the condition it targets.