Keratinocytes are the skin cells at the heart of psoriasis. In someone with the condition, these cells multiply and move to the surface far faster than normal, piling up into the thick, scaly patches that define the disease. This rapid turnover is essentially what's happening underneath every plaque.

The Koebner phenomenon describes new psoriasis patches forming exactly where the skin has been injured — a scratch, a cut, even a sunburn. It shows how something as small as a minor scrape can trigger new disease activity in someone already living with psoriasis. It's a good reason to be gentle with the skin.