Video: Psoriasis

Animation of the skin layers and normal function of skin cells showing the immune response and what goes wrong in the skin cells causing psoriasis.

Your skin is composed of 3 layers. The outer layer, or epidermis, is made up of cells called keratinocytes. They begin to form at the bottom of the epidermis. They divide, pushing already-formed cells upwards. By the time the cells reach the skin's surface, they are no longer alive. These are the dead skin cells we constantly shed. This process usually takes about 30 days. In psoriasis, it takes only 3 to 4. And instead of falling off, the dead cells pile up.

Psoriasis results from a problem with a type of white blood cell in the immune system called a T cell. T cells normally detect and fight off foreign substances. In psoriasis, T cells attack healthy skin cells and trigger an immune response.

Blood vessels in the skin get larger. Other white blood cells enter the epidermis. The T cells release messenger chemicals called cytokines, which tell the keratinocytes to reproduce and mature much faster than normal.

New skin cells form and move to the surface too quickly. They build up as the thick, scaly patches on the skin's surface that characterize psoriasis.

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