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# Psoriasis and seborrheic dermatitis

- Lesion

Psoriasis is a lifelong immune disease with thick silvery plaques on elbows, knees and scalp. Seborrheic dermatitis is milder greasy scaling.

Updated Nov 12, 2026

## Key points

- Genetics: HLA-C*06:02 is the strongest risk allele; variants in IL23R, IL12B and CARD14.
- Immunology: dendritic cells make IL-23; Th17 cells make IL-17 and IL-22, driving keratinocyte proliferation and neutrophil recruitment.
- Histology: regular (uniform) acanthosis with elongated rete ridges, thin suprapapillary plates, absent granular layer, confluent parakeratosis, neutrophils in the stratum corneum (Munro microabscesses) and spinous layer (spongiform pustules of Kogoj), dilated papillary vessels.
- Auspitz sign: pinpoint bleeding when scale is removed; Koebner phenomenon: lesions at sites of trauma.
- Nail pitting and onycholysis; psoriatic arthritis in a substantial minority.
- Systemic links: metabolic syndrome, cardiovascular disease, depression.

## What it is

Psoriasis is a common chronic inflammatory disease driven by the IL-23 and Th17 axis, in which keratinocytes proliferate very fast and mature poorly. It produces sharply demarcated salmon-pink plaques with silvery scale and can involve nails and joints. Seborrheic dermatitis is a frequent inflammatory scaling of sebaceous-rich skin (scalp, face, chest), linked to Malassezia yeast and more severe in HIV and Parkinson disease.

## Structure

- Gross: well-demarcated pink plaques with silvery-white scale on extensor surfaces, scalp, sacrum and nails.
- Micro: as listed above; keratinocyte turnover is greatly accelerated.
- Seborrheic dermatitis: mild psoriasiform hyperplasia with spongiosis and parakeratosis around follicular openings (shoulder parakeratosis).

## Step by step

- A susceptible person (HLA-C*06:02) meets a trigger: trauma, streptococcal infection, drugs (lithium, beta-blockers), stress.
- Keratinocyte damage releases antimicrobial peptides and nucleic acids that activate dendritic cells.
- Dendritic cells produce IL-23 and TNF, expanding Th17 cells.
- IL-17 and IL-22 drive keratinocyte hyperproliferation and chemokines that attract neutrophils.
- A self-sustaining loop forms the plaque; drugs blocking TNF, IL-23 or IL-17 break it.

## Clinical notes

- Guttate psoriasis follows streptococcal pharyngitis in young people.
- Severe or sudden psoriasis and seborrheic dermatitis can be the first sign of HIV infection.
- Seborrheic dermatitis is cradle cap in infants and dandruff in adults.

## Memory aid

Psoriasis is a factory speeded up by IL-17: the bricks (keratinocytes) arrive unfinished (parakeratosis) and neutrophils camp on the roof (Munro).

## Test yourself

Think of the answer, then tap the question to see it.

## What are Munro microabscesses?

Collections of neutrophils within the parakeratotic stratum corneum in psoriasis.

## Which cytokine axis drives psoriasis?

The IL-23 and Th17 (IL-17, IL-22) axis, with TNF.

## What is the Auspitz sign?

Pinpoint bleeding when psoriatic scale is removed, from dilated vessels under thin suprapapillary plates.

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## Elsewhere in DurgiPedia

- [Interleukin-17](https://durgical.com/en/immunology/interleukin-17/)
- [IL-12 and IL-23](https://durgical.com/en/immunology/il-12-and-il-23/)
- [HLA associations with disease](https://durgical.com/en/immunology/hla-associations-with-disease/)

## Connections

## Related 3

- [Skin pathology: terms and reaction patterns](https://durgical.com/en/general-pathology/skin-pathology-terms-and-reaction-patterns/)
- [Eczematous dermatitis](https://durgical.com/en/general-pathology/eczematous-dermatitis/)
- [Lichen planus and interface dermatitis](https://durgical.com/en/general-pathology/lichen-planus-and-interface-dermatitis/)

## Related conditions 3

- [Psoriasis](https://durgical.com/en/conditions/psoriasis/)
- [Seborrheic dermatitis](https://durgical.com/en/conditions/seborrheic-dermatitis/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)

## Microorganisms 1

- [Malassezia](https://durgical.com/en/microbiology/malassezia/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Psoriasis](https://medlineplus.gov/psoriasis.html) (MedlinePlus, US National Library of Medicine, 2025); [Psoriatic arthritis](https://medlineplus.gov/genetics/condition/psoriatic-arthritis/) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/psoriasis-and-seborrheic-dermatitis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/psoriasis-and-seborrheic-dermatitis/