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What Causes Psoriasis?

WebMD Feature

Psoriasis turns your skin cells into Type A overachievers: They mature about five times faster than cells in normal skin. And unlike normal skin cells, which naturally slough off, these cells pile up on the skin's surface almost faster than snow on a snowdrift. Why does this happen? What makes these cells go a little haywire?

The cause of psoriasis remains largely a mystery. But it's likely that the interplay between genes and triggers is a big part of the story. Researchers believe that psoriasis develops when something mistakenly triggers the immune system. And in psoriatic arthritis -- psoriasis that affects the joints -- both genetics and environmental factors seem to play a role.

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Psoriasis is a common skin disorder that forms thick, red, bumpy patches covered with silvery scales. They can pop up anywhere, but most appear on the scalp, elbows, knees, and lower back. Psoriasis can't be passed from person to person. It does sometimes happen in members of the same family. It usually appears in early adulthood. For most people, it affects just a few areas. In severe cases, it can cover large parts of the body. The rashes can heal and then come back throughout a person's...

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The Immune System: The Root Cause of Psoriasis

"Originally, psoriasis was thought of as a hyperproliferative disease," says Colby Evans, MD, who is a dermatologist and psoriasis expert in Austin, Texas. "In other words, it was thought the skin was simply making too much skin. So, most of the treatments were directed at that problem."

But in 1979, a chance event shed new light on the cause. Researchers noticed that people undergoing bone marrow transplants for other conditions also became free of psoriasis when they were given a drug to suppress the immune system.

"So we now know that psoriasis is an abnormality or malfunction of the immune system," Evans tells WebMD. "And, specifically, we know that T cells, a type of white blood cell, are at the root of it. They are overgrowing and attacking the area of skin where the psoriasis is located. When you biopsy psoriasis and look at it with a microscope, you'll see many T cells underneath the plaque."

Instead of protecting the body from invaders as it normally does, the immune system in someone with psoriasis promotes inflammation and a speedy growth of skin cells. In a normal immune response, the body releases proteins called cytokines, which the immune system uses to send messages. But with psoriasis, cytokines get their signals mixed, so to speak. Then skin cells pile up, the skin thickens, and the area becomes red as inflammation and blood flow increase.

"A lot of the new treatments are targeted at trying to contain or inactivate T cells that are causing the hyperproliferation in the skin," Evans says. What triggers the activation of T cells, or keeps them on high alert, is not well understood, he adds. But genes may have something to do with it.

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