An estimated 7.5 million Americans (2.2% of the population) have psoriasis. Psoriasis is a chronic skin disorder in which there are periodic flare-ups of sharply defined red patches, covered by a silvery, flaky surface. The main disease activity leading to psoriasis occurs in the epidermis, the top five layers of the skin.
The process starts in the basal (bottom) layer of the epidermis, where keratinocytes are made. Keratinocytes are immature skin cells that produce keratin, a tough protein that helps form hair, nails, and skin. In normal cell growth, keratinocytes grow and move from the bottom layer to the skin's surface and shed unnoticed. This process takes about a month.
In people with psoriasis, the keratinocytes multiply very rapidly and travel from the basal layer to the surface in about 4 days. The skin cannot shed these cells quickly enough, so they build up, leading to thick, dry patches, or plaques. Silvery, flaky areas of dead skin build up on the surface of the plaques before being shed. The skin layer underneath (dermis), which contains the nerves and blood and lymphatic vessels, becomes red and swollen.
Review Date: 10/21/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.