Psoriasis ( /səˈraɪ.əsɨs/) is an autoimmune disease that affects the skin. It occurs when the immune system mistakes the skin cells as a pathogen, and sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious. However, psoriasis has been linked to an increased risk of stroke, and treating high blood lipid levels may lead to improvement. There are five types of psoriasis: plaque, guttate, inverse, pustular, and erythrodermic. The most common form, plaque psoriasis, is commonly seen as red and white hues of scaly patches appearing on the top first layer of the epidermis (skin). Some patients, though, have no dermatological signs or symptoms. The name psoriasis is from Ancient Greek, meaning roughly "itching condition" (psora "itch" + -sis "action, condition").
In plaque psoriasis, skin rapidly accumulates at these sites, which gives it a silvery-white appearance. Plaques frequently occur on the skin of the elbows and knees, but can affect any area, including the scalp, palms of hands and soles of feet, and genitals. In contrast to eczema, psoriasis is more likely to be found on the outer side of the joint.
The disorder is a chronic recurring condition that varies in severity from minor localized patches to complete body coverage. Fingernails and toenails are frequently affected (psoriatic nail dystrophy) and can be seen as an isolated sign. Psoriasis can also cause inflammation of the joints, which is known as psoriatic arthritis. Between 10—30% of all people with psoriasis also have psoriatic arthritis.
The cause of psoriasis is not fully understood, but it is believed to have a genetic component and local psoriatic changes can be triggered by an injury to the skin known as the Koebner phenomenon. Various environmental factors have been suggested as aggravating to psoriasis, including stress, withdrawal of systemic corticosteroid, as well as other environmental factors, but few have shown statistical significance. There are many treatments available, but because of its chronic recurrent nature, psoriasis is a challenge to treat. Withdrawal of corticosteroids (topical steroid cream) can aggravate the condition due to the 'rebound effect' of corticosteroids.