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Psoriasis · Pattern story 07

Why a scratch can turn into a psoriasis plaque, and why most scratches don't

A cat scratch on a clear forearm bleeds a little and heals normally. Two weeks later the thin pink line thickens, turns deep red and grows silvery scale, tracing the scratch exactly. It feels like a rule. The controlled studies say it is a tendency, and a selective one.

By Laxmanna · Updated 2026-10-21 · 7 min read · 16 sources

1–3 weeks

between injury and a visible plaque in controlled human experiments. Nothing visible happens in the first days.[2]

72.5%

of 62 people with psoriasis developed zero plaques across ten standardised needle pricks in the largest controlled test.[4]

0 of 38

cosmetic laser treatments on clear facial skin produced a new plaque in a retrospective record review.[11]

The plaque that follows the injury

Heinrich Koebner described it in 1876: in a person with an existing skin disease, trauma to previously unaffected skin can produce that same disease at the site of the injury.[1] In psoriasis it has been reproduced under controlled conditions with tape stripping, needle pricks, scraping and suction blisters.[2][5] The new plaque appears one to three weeks after the injury and conforms to its shape: a line for a scratch, a curve for an incision.[2]

How many people with psoriasis experience it depends on how you ask. When adults are asked to recall, roughly 28 to 33% say yes.[3] In clinic records of children, documented prevalence is closer to 15%.[9] Recall tends to inflate, and the number you may have seen quoted, '11% to 75%', traces back to secondary reviews with no underlying dataset.[13] Traceable primary experimental figures cluster between about 22% and 60% depending on the depth and type of injury.[4][5][6]

Most injuries don't

This is the part that gets lost. In controlled human trials, the majority of test injuries do not produce a plaque. In the largest standardised study, 62 people with psoriasis each received ten needle pricks on the forearm; 72.5% developed no lesion at any site.[4] Among those who did react, 16 of 17 reacted at some pricks but not others.[4] An older study found the same person could react to an unroofed suction blister yet show nothing after tape stripping.[5] The idea that Koebner is an all-or-none trait, where you either have it or you don't, is disproven.

Depth matters: plaque formation seems to need both rupture of the epidermal barrier and inflammatory signalling in the deeper dermis.[6] Proposed risk factors such as active flaring disease, younger age of onset or carrying the HLA-Cw6 gene have given mixed or contradictory results across studies.[7][8]

Needle prick, 10 sites: no plaque anywhere72.5%
Tape stripping: positive22–58%
Adult recall: 'yes, it has happened to me'~33%
Paediatric clinic records~15%
The answer to 'how common is it?' depends on how it was measured.[3][4][5][9]

What happens in the skin during the delay

Serial biopsies after tape stripping in people who do koebnerise show an ordered cascade. Within two hours, injured keratinocytes release alarm signals and mast cells activate IL-6.[2][6] By 24 hours, keratinocyte nerve growth factor and TNF-α surge, yet there are still no T cells in the epidermis.[2] Over days three to seven, myeloid immune cells accumulate in the dermis. Only at weeks two to three do T cells cross into the epidermis, and that is when a visible papule forms.[6]

The reason clear skin can do this at all is that in psoriasis, 'uninvolved' skin is not truly uninvolved. Grafts of clear psoriatic skin onto immunodeficient mice spontaneously convert into plaques, and resolved lesions keep disease-specific T-cell clones.[10][12] Injury does not create the disease. It acts as a localiser, deciding where a plaque lands in skin that is already primed.

The finer molecular details (plasmacytoid dendritic cells, type I interferon, LL37, stretch-sensing channels) come mostly from mouse and cell models. No human study has identified the switch that decides who koebnerises and who does not.[10]

Tattoos, lasers and surgery

In a multicentre study of 2,053 adults with psoriasis, 894 of whom had tattoos, 6.6% of tattoos had some local complication and there were no severe adverse events.[14] That 6.6% is a composite that includes swelling, itching and ink allergy as well as Koebner reactions, so it is not a Koebner rate.[14] Self-selected online surveys of tattooed people with psoriasis report Koebner reactions along ink lines in anywhere from 5% to 28%, but those surveys are heavily skewed (89% female in one) and cannot give an individual their odds.[15]

Cosmetic lasers on clear, non-inflamed facial skin produced no new plaques in a retrospective review of 38 treatments.[11] Lasers applied to active plaques are a different matter, and so is medication: there is no trial testing whether biologics or systemic drugs protect against koebnerisation, and in one case report, stopping methotrexate three months before surgery was followed by a severe pustular flare.[16]

What this means for you

  • Injury localises a plaque in skin that is already primed; it does not cause psoriasis.
  • Most cuts, scratches and pricks heal normally, even in people who have koebnerised before. One reaction does not predict the next.
  • The one-to-three-week delay matters for tracking. If a plaque appears somewhere new, look back two to three weeks for an injury, friction or sunburn at that spot.
  • Decisions about tattoos, laser treatments, surgery, and any medication timing around them belong in a conversation with your dermatologist. Group complication rates cannot give you a personal probability.
  • Do not stop or pause systemic treatment before a procedure on your own.[16]

Questions for your dermatologist

  1. Have I had Koebner reactions before, and does that change anything about how I should treat minor injuries?
  2. I am considering a tattoo / laser treatment / surgery. What does my current disease activity mean for that, and how should my treatment be handled around it?
  3. If a new plaque appears along a scratch or incision, what is the right first step?
  4. How do I tell a Koebner plaque from an infection or an ink reaction?
  5. Are there sites on my body where you would be more cautious about procedures?

When it is not Koebner

True Koebner is a red, raised, silvery-scaled plaque appearing 1 to 3 weeks after injury and following its outline. Infection looks different: rapid onset within hours to days, spreading redness and warmth, increasing pain, pus, or fever, and needs urgent medical evaluation. Ink allergy or irritation brings acute itching, hives or eczema-like patches within hours to days. Warts or molluscum spreading along a scratch line are infectious 'pseudo-Koebner', not psoriasis.[1] Widespread sterile pustules with fever after stopping a systemic medication is a rebound flare and an emergency.[16]

Things people hear, and what the evidence says

"Every cut or tattoo will give me a plaque."
In the largest controlled test, nearly three-quarters of people developed no plaque from ten needle pricks.[4]
"Trauma causes psoriasis."
Trauma decides where a plaque appears in someone who already has the disease. Clear psoriatic skin is primed; injury is the localiser.[10][12]
"If I'm on a biologic, I'm protected from Koebner."
No trial has tested that. Biologics clear existing plaques; procedural protection is an assumption.[14][16]
"Tattoos have a 6.6% chance of triggering psoriasis."
That figure is a composite of all local tattoo complications, including itching and ink allergy, not a Koebner rate.[14]
"I reacted once, so I always will."
Reactions are site- and injury-specific. People react at some sites and not others, and to some injuries and not others.[4][5]

Sources

Primary human studies are marked "primary"; reviews, guidelines and models are labelled. Where a source is an abstract only, that is stated.

  1. Narrative review of the Koebner phenomenon in psoriasis, including history and look-alike reactions. PMC 2019 — review. https://pmc.ncbi.nlm.nih.gov/articles/PMC6893164/
  2. Raychaudhuri SP et al. Role of nerve growth factor in the Koebner phenomenon: experimental tape stripping with serial biopsies, culture and xenograft. Am J Pathol 2008 — primary. https://pmc.ncbi.nlm.nih.gov/articles/PMC2276420/
  3. Melski JW, Bernhard JD, Stern RS. The Koebner (isomorphic) response in psoriasis: cross-sectional history in a PUVA-trial cohort. Arch Dermatol 1983 — abstract. https://pubmed.ncbi.nlm.nih.gov/6603199/
  4. Kalayciyan A et al. An experimental study of the Koebner phenomenon with standardised needle pricks in 62 patients. Dermatology 2007 — abstract. https://pubmed.ncbi.nlm.nih.gov/17684372/
  5. Powles AV et al. Experimental Koebner induction by tape stripping and suction blisters. Acta Derm Venereol 1990 — abstract. https://pubmed.ncbi.nlm.nih.gov/1967869/
  6. Suttle MM et al. Experimentally induced psoriatic lesions: serial biopsies after tape stripping. Clin Exp Immunol 2012 — primary. https://pmc.ncbi.nlm.nih.gov/articles/PMC3445008/
  7. Matović et al. Clinical cohort testing proposed modifiers of the Koebner response. Med Pregl 2000 — abstract. https://pubmed.ncbi.nlm.nih.gov/11089373/
  8. Genetic association study of Koebner response and HLA-Cw6 in a familial cohort. 2002 — abstract. https://pubmed.ncbi.nlm.nih.gov/11841557/
  9. Oostveen AM et al. Prospective paediatric psoriasis cohort including documented Koebner prevalence. Acta Derm Venereol 2014 — primary. https://www.medicaljournals.se/acta/content/html/10.2340/00015555-1810
  10. Boyman O et al. Spontaneous development of psoriasis in a new animal model (xenografts of clear psoriatic skin). J Exp Med 2004 — primary. https://pubmed.ncbi.nlm.nih.gov/14981113/
  11. Sachsman SM et al. Retrospective review of cosmetic laser treatment of non-lesional skin in psoriasis patients. Dermatol Surg 2016 — abstract. https://pubmed.ncbi.nlm.nih.gov/26981721/
  12. Matos TR et al. Clinically resolved psoriatic lesions contain psoriasis-specific IL-17-producing T cell clones. J Clin Invest 2017 — primary. https://pubmed.ncbi.nlm.nih.gov/28945199/
  13. Koebner Phenomenon. StatPearls, NCBI Bookshelf — reference review (source of widely quoted secondary ranges). https://www.ncbi.nlm.nih.gov/books/NBK553108/
  14. Grodner C et al. Tattoo complications in adults with psoriasis: multicentre cross-sectional study of 2,053 patients. JEADV 2020 — abstract. https://pubmed.ncbi.nlm.nih.gov/31568596/
  15. Rogowska P et al. Tattoos and psoriasis: online survey of 150 tattooed patients. Clin Cosmet Investig Dermatol 2022 — primary. https://pmc.ncbi.nlm.nih.gov/articles/PMC8995150/
  16. Severe pustular flare after stopping methotrexate before surgery: case report (Vaggu et al. 2012), as discussed in the Koebner literature review. PMC — case report. https://pmc.ncbi.nlm.nih.gov/articles/PMC3626811/

Written by Laxmanna · Research-led health writer. Every explainer on Unflame starts from a structured evidence dossier: primary studies retrieved, claims tabulated, contradictions logged, and wording checked against what the sources can support. How the dossiers are built.

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