What Causes Psoriasis?

Normal skin has two layers: an outer layer called the epidermis and an inner layer called the dermis. Skin cells are created in the dermis and move up through the epidermis to the skin surface. Skin cells are continually dying and being replaced. Normally, skin cells mature and shed in about a month.

In psoriasis, however, the normal cycle of replacing old skin cells with new ones becomes unbalanced. Skin cells reproduce many times faster than normal and live only three to four days. The dead cells build up on the skin, forming thick, flaky patches called plaques. The redness in skin plaques is caused by increased blood supply to the rapidly multiplying skin cells.

Why skin cells begin to multiply abnormally is not well understood. Heredity is one factor, however. In about one third of cases, psoriasis is inherited. It is estimated that if a child has one parent with psoriasis, the child’s chances of getting the disease are between one in 10 and one in four. If both parents have psoriasis, a child’s chances of getting the disease are about 50-50.

Evidence from recent research studies also strongly suggests that psoriasis is a disorder of the immune system. Normally, the immune system defends the body from infection by bacteria, viruses, and other invaders. Sometimes, however, the immune system makes a mistake and attacks the cells, tissues, and organs of a person’s own body. When this happens, the resulting disease is called an autoimmune disease. Many autoimmune diseases run in families.

The theory that psoriasis is an autoimmune disease is based on the fact that scientists have found abnormally large numbers of t cells (a type of white blood cell) in the red, flaky skin patches of people with psoriasis. T cells are the infantry of the immune system. When the body senses a need to defend itself against infection, it first makes and then releases millions of t cells to fight off the invaders. Some t cells are normally found in skin. The presence of abnormally large numbers of t cells in skin affected by psoriasis suggests that the immune system is attacking the skin by mistake.

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