Causes
The cause of psoriasis is related to the immune system, and more specifically, a type of white blood cell called a T lymphocyte or T cell. Normally, T cells travel throughout the body to detect and fight off foreign substances, such as viruses or bacteria. If you have psoriasis, however, the T cells attack healthy skin cells by mistake as if to heal a wound or to fight an infection.
Overactive T cells trigger other immune responses including dilation of blood vessels in the skin around the plaques and an increase in other white blood cells that can enter the epidermis. These changes result in an increased production of both healthy skin cells and more T cells and other white blood cells. What results is an ongoing cycle in which new skin cells move to the outermost layer of skin too quickly — in days rather than weeks. Dead skin and white blood cells can't slough off quickly enough and build up in thick, scaly patches on the skin's surface. This usually doesn't stop unless treatment interrupts the cycle.
Just what causes T cells to malfunction in people with psoriasis isn't entirely clear, although researchers think genetic and environmental factors both play a role.
Psoriasis triggers
Psoriasis typically starts or worsens because of a trigger that you may be able to identify and avoid. Factors that may trigger psoriasis include:
Infections, such as strep throat or thrush
Injury to the skin, such as a cut or scrape, bug bite, or a severe sunburn
Stress
Cold weather
Smoking
Heavy alcohol consumption
Certain medications — including lithium, which is prescribed for bipolar disorder; high blood pressure medications such as beta blockers; antimalarial drugs; and iodides
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