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What Is Psoriasis?

What Is Psoriasis - Overview

Psoriasis is a common and chronic skin disease. A person with psoriasis generally has patches of raised red skin with thick silvery scales. The affected skin may be red and scaly or have pustules, depending on the type of psoriasis the individual has.

Psoriasis is usually found on the arms, legs, trunk, nails, or scalp, but psoriasis may be found on any part of the skin. The most commonly affected areas are the knees and elbows.

Plaque psoriasis on the elbow. Plaque psoriasis is the most common type of psoriasis. Image courtesy of Hon Pak, MD.

Psoriasis is not an infection and therefore is not contagious. Touching the affected skin and then touching someone else will not transmit psoriasis. However, the red scaly skin can become infected, especially when there are splits in the skin.

The immune system plays a key role in psoriasis. The immune system makes white blood cells that protect the body from infection. In psoriasis, the T cells (a type of white blood cell) abnormally trigger inflammation in the skin. These T cells also cause skin cells to grow faster than normal and to pile up in raised patches on the outer surface of the skin.

Plaque psoriasis on the back. Image courtesy of Hon Pak, MD.

Psoriasis can be inherited. Some people carry genes that make them more likely to develop psoriasis. Just because a person has genes that would make him more likely to have psoriasis doesn’t mean he will have the disease. About one third of people with psoriasis have at least one family member with the disease. Certain factors trigger psoriasis to flare up in those who have the genes.

Environmental factors such as smoking, exposure to sun, and alcoholism may affect how often psoriasis occurs and how long the flare-ups last. Injury to the skin has been known to trigger psoriasis. For example, a skin infection, skin inflammation, or even excessive scratching can trigger psoriasis. A number of medications have been shown to aggravate psoriasis.

Psoriasis flare-ups can last for weeks or months. The psoriasis goes away for a time and then returns.

Plaque psoriasis is the most common type of psoriasis and is characterized by red skin covered with silvery scales and inflammation. Patches of circular- or oval-shaped red plaques that itch or burn are typical of plaque psoriasis.

Plaque psoriasis with fissures, which are splits in the skin. Fissures usually occur where the skin bends (joints). The skin may bleed and is more susceptible to infection. Image courtesy of Hon Pak, MD.

Approximately 1-2% of people in the United States, or about 5.5 million, have plaque psoriasis. Up to 10% of people with plaque psoriasis also have psoriatic arthritis. Individuals with psoriatic arthritis have inflammation in their joints and may have other arthritis symptoms. Guttate psoriasis can occur after a streptococcal infection (bacterial infection). Guttate psoriasis looks like little red drops on the skin.

Guttate psoriasis. Red droplike lesions are found on the skin. Image courtesy of Hon Pak, MD.

Sometimes plaque psoriasis can evolve into more severe disease, such as pustular or erythrodermic psoriasis. In pustular psoriasis, the red areas on the skin contain blisters with pus. In erythrodermic psoriasis, a wide area of red and scaling skin is typical, and it may be itchy and painful.

Pustular psoriasis. Note the clearly defined, raised bumps on the skin that are filled with pus (pustules). The skin under and around these bumps is reddish. Image courtesy of Hon Pak, MD.