Psoriasis is a long-lasting, noncontagious autoimmune disease characterized by raised areas of abnormal skin.

These areas are red, or purple on some people with darker skin, dry, itchy, and scaly. Psoriasis varies in severity from small, localized patches to complete body coverage.

Injury to the skin can trigger psoriatic skin changes at that spot, which is known as the Koebner phenomenon.

The five main types of psoriasis are plaque, guttate, inverse, pustular, and erythrodermic.

• Plaque psoriasis, also known as psoriasis vulgaris, makes up about 90% of cases. It typically presents as red patches with white scales on top.Areas of the body most commonly affected are the back of the forearms, shins, navel area, and scalp.
• Guttate psoriasis has drop-shaped lesions.
• Pustular psoriasis presents as small, noninfectious, pus-filled blisters.
• Inverse psoriasis forms red patches in skin folds.
• Erythrodermic psoriasis occurs when the rash becomes very widespread, and can develop from any of the other types.

Psoriasis is generally thought to be a genetic disease that is triggered by environmental factors.

Psoriatic arthritis is a form of chronic inflammatory arthritis that has a highly variable clinical presentation and frequently occurs in association with skin and nail psoriasis.It typically involves painful inflammation of the joints and surrounding connective tissue, and can occur in any joint, but most commonly affects the joints of the fingers and toes.

Psoriasis is characterized by an abnormally excessive and rapid growth of the epidermal layer of the skin.

Skin cells are replaced every 3–5 days in psoriasis rather than the usual 28–30 days.

A diagnosis of psoriasis is usually based on the appearance of the skin. Skin characteristics typical for psoriasis are scaly, erythematous plaques, papules, or patches of skin that may be painful and itch. No special blood tests or diagnostic procedures are usually required to make the diagnosis.