Acne Vulgaris Pathogenesis
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Direct answer
Acne vulgaris is a multifactorial disease of the pilosebaceous unit driven by four interacting processes: androgen-stimulated sebum production, follicular hyperkeratinisation, colonisation by Cutibacterium acnes and inflammation. Androgens push sebaceous glands to overproduce sebum, while retained keratin blocks the follicle and forms microcomedones. Cutibacterium acnes then amplifies the process through lipases and immune activation, so all four factors must be understood together.
What you must remember
- Four factors: sebum excess, follicular hyperkeratinisation, Cutibacterium acnes colonisation and inflammation, each necessary for clinically significant acne.
- Androgen drive: testosterone and DHEA-S stimulate sebaceous glands, with local conversion to dihydrotestosterone by 5-alpha reductase in the pilosebaceous unit.
- Sebum composition: reduced linoleic acid in sebum is linked with abnormal keratinisation of the follicular lining and comedo formation.
- Microcomedone: the earliest lesion, a keratin and sebum plug within a dilated follicle, which precedes visible inflammation and evolves into open or closed comedones.
- Cutibacterium acnes: a commensal of healthy skin whose lipases release irritant free fatty acids and whose products activate innate immunity through Toll-like receptor 2.
- Inflammation: cytokine release and neutrophil infiltration convert the plugged follicle into papules, pustules and nodules, and inflammation begins early rather than as a late event.
- Typical sites: face, upper back and chest, which carry the highest density of sebaceous follicles.
From androgen signal to inflamed lesion
A 17-year-old boy develops closed comedones on the forehead and chin with a few tender papules within months of puberty. The sequence explains why a single factor is never enough, and each step creates the conditions for the next.
- Androgen signal: Rising androgens at puberty and adrenarche stimulate sebaceous glands, and local conversion to dihydrotestosterone increases the drive. Most patients have normal androgen levels, but their follicles respond more strongly.
- Sebum overproduction: Enlarged sebaceous glands produce more sebum, changing its lipid composition and reducing linoleic acid. This alteration sets up the follicle for abnormal keratinisation.
- Follicular hyperkeratinisation: Keratinocytes in the follicular lining shed abnormally and stick together, blocking the outflow of sebum. A microcomedone forms before any visible inflammation appears.
- Bacterial amplification: Cutibacterium acnes multiplies in the sebum-rich, anaerobic follicle, and its lipases release free fatty acids that irritate the lining. The organism's products trigger innate immune receptors.
- Inflammatory cascade: Cytokines and chemotactic signals draw neutrophils and lymphocytes into the follicle. The inflamed follicle ruptures into papules, pustules or nodules, and the scarring risk rises with deeper inflammation.
Where the exam tries to catch you
Acne pathogenesis questions test whether the four factors are kept distinct and whether the androgen story is understood correctly.
- Trap: blaming dirt or poor hygiene for acne. Correct: the causes are internal, including sebum, keratinisation and immune factors, and washing does not remove the follicular plug.
- Trap: attributing acne to Cutibacterium acnes alone. Correct: the organism is a commensal of healthy skin, and disease needs the sebum, keratinisation and inflammatory factors as well.
- Trap: assuming every patient has high circulating androgens. Correct: most patients have normal androgen levels, because the sebaceous glands show a heightened local response to androgens.
- Trap: regarding inflammation as a late complication of acne. Correct: inflammation begins early in the microcomedone, which is why anti-inflammatory treatment forms part of therapy.
Frequently asked questions
Which four factors drive acne vulgaris pathogenesis?
The four factors driving acne vulgaris pathogenesis are sebum overproduction, follicular hyperkeratinisation, colonisation by Cutibacterium acnes and inflammation. Each is necessary for clinically significant acne, and treatments are chosen to target one or more of them.
What role does Cutibacterium acnes play in acne vulgaris?
Cutibacterium acnes is a commensal of healthy skin that contributes to acne by producing lipases that release irritant free fatty acids and by activating innate immunity. It is not sufficient on its own, because disease also requires sebum excess and abnormal keratinisation.
Why do many acne patients have normal androgen levels?
Many acne patients have normal androgen levels because their sebaceous glands respond more strongly to circulating androgens, including local conversion to dihydrotestosterone. Acne therefore reflects heightened follicular sensitivity rather than necessarily elevated hormone levels.
What is a microcomedone and why does it matter?
A microcomedone is the earliest lesion of acne, a keratin and sebum plug within a dilated follicle that forms before visible inflammation. It matters because it is the precursor from which open and closed comedones, papules and pustules develop.