What causes psoriasis?

No one knows exactly what causes psoriasis. However, it is understood that the immune system and genetics play major roles in its development. Most researchers agree that the immune system is somehow mistakenly triggered, which causes a series of events, including acceleration of skin cell growth. A normal skin cell matures and falls off the body in 28 to 30 days. A skin cell in a patient with psoriasis takes only 3 to 4 days to mature and instead of falling off (shedding), the cells pile up on the surface of the skin, forming psoriasis lesions.

Scientists believe that at least 10 percent of the general population inherits one or more of the genes that create a predisposition to psoriasis. However, only 2 percent to 3 percent of the population develops the disease. Researchers believe that for a person to develop psoriasis, the individual must have a combination of the genes that cause psoriasis and be exposed to specific external factors known as “triggers.”

Learn more about genetic and immune system involvement in psoriasis and psoriatic arthritis.
Psoriasis triggers

Psoriasis triggers are not universal. What may cause one person’s psoriasis to become active, may not affect another. Established psoriasis triggers include:
Stress

Stress can cause psoriasis to flare for the first time or aggravate existing psoriasis. Relaxation and stress reduction may help prevent stress from impacting psoriasis.
Injury to skin

Psoriasis can appear in areas of the skin that have been injured or traumatized. This is called the Koebner [KEB-ner] phenomenon. Vaccinations, sunburns and scratches can all trigger a Koebner response. The Koebner response can be treated if it is caught early enough.
Medications

Certain medications are associated with triggering psoriasis, including:

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Lithium: Used to treat manic depression and other psychiatric disorders. Lithium aggravates psoriasis in about half of those with psoriasis who take it.
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Antimalarials: Plaquenil, Quinacrine, chloroquine and hydroxychloroquine may cause a flare of psoriasis, usually 2 to 3 weeks after the drug is taken. Hydroxychloroquine has the lowest incidence of side effects.
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Inderal: This high blood pressure medication worsens psoriasis in about 25 percent to 30 percent of patients with psoriasis who take it. It is not known if all high blood pressure (beta blocker) medications worsen psoriasis, but they may have that potential.
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Quinidine: This heart medication has been reported to worsen some cases of psoriasis.
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Indomethacin: This is a nonsteroidal anti-inflammatory drug used to treat arthritis. It has worsened some cases of psoriasis. Other anti-inflammatories usually can be substituted. Indomethacin's negative effects are usually minimal when it is taken properly. Its side effects are usually outweighed by its benefits in psoriatic arthritis.

Other triggers

Although scientifically unproven, some people with psoriasis suspect that allergies, diet and weather trigger their psoriasis. Strep infection is known to trigger guttate psoriasis.