What Causes Psoriasis?

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Updated: 11/03/2011 3:11 pm

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.

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.

Causes of Psoriasis: The Role of Genes

One out of 10 people have genes that make them more likely to get psoriasis. But most people with psoriasis don't report having a relative with the disease. So while there are clearly genetic connections, they're not exactly clear-cut.

"It's not a simple genetic program like green eyes or blue eyes, where it passes through a family in a specific pattern," Evans says. "And it isn't a strictly genetic disease the way cystic fibrosis is." Think of it more as tendency toward disease, possibly brought on by a variety of genetic changes. For example, errors in DNA chemicals may lead to missing, misshapen, or misplaced proteins.

To actually develop psoriasis, you must have a combination of different genes. Researchers are still working to identify all of them. Once this happens, the causes of psoriasis will become much clearer.

A significant percentage of people have genes that predispose them to the disease. But only a fraction -- maybe 2% or 3% of the population -- develop psoriasis. It's likely that the tendency toward psoriasis requires certain triggers to get fully set into motion. "Whether that is environmental triggers or infections or other types of triggers is unknown," Evans says.

Causes of Psoriasis: The Role of Triggers

A wide range of triggers is linked with the onset or flare-up of psoriasis. These vary from person to person, but may include:

  • Stress
  • Medications
  • Infections
  • Injuries

"We know that injuries bring on psoriasis at the site of the injury," Evans says. "For example, if someone has a surgery, they will sometimes get psoriasis along the line of the scar." Called the Koebner phenomenon, this can also occur following vaccinations, bug bites, sunburns, or cuts. As many as half those with psoriasis experience this phenomenon. It provides a strong argument for protecting your skin, especially during summer months.

Some medications are also more likely to trigger a flare of psoriasis. They include:

  • Lithium, used to treat certain psychiatric disorders
  • Antimalarial medications
  • Certain high blood pressure or heart medications such as inderal or quinidine
  • Indomethacin, a nonsteroidal anti-inflammatory drug

"There can also be an onset after an infection such as strep throat, especially in young people," Evans says. Fortunately, treating the infection may help the flare-up to cool down.

Other triggers linked to psoriasis include allergies, diet, dry and cold weather, smoking, and heavy use of alcohol. But much more research is needed to complete the puzzle that makes up this complex disease.

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