Psoriasis is a common skin disease that affects the life cycle of skin cells. Normally, new cells take about a month to move from the lowest skin layer (where they're produced) to the outermost layer (where they die and flake off). With psoriasis, the entire life cycle takes only days. As a result, cells build up rapidly, which then form silvery scales and dry, itchy, red patches. The cause is related to the immune system, to be specific, a type of white blood cell called a T cell. T cells travel throughout the body to detect and fight off foreign substances (such as bacteria or viruses). In people with psoriasis, the T cells attack healthy skin cells by mistake as if to heal a wound or fight an infection. Now, what causes the T cells to malfunction in people isn't clear, but researchers believe genetic and enviornmental factors both play a role.