Psoriasis is not a cancer, but does seem to be related to the immune system. Psoriasis is a common, chronic skin disease that affects the life cycle of skin cells. Psoriasis causes cells to build up rapidly on the surface of the skin, forming thick silvery scales and itchy, dry, red patches that are sometimes painful. Itching may be severe, and scratching the rash can make it even itchier and cause more inflammation. Once the skin barrier is broken, the skin can become infected by bacteria, especially Staphylococcus aureus, which commonly live on the skin. It is a common, chronic, scaly rash that affects people of all ages (about 2% of the population). There is a genetic predisposition to psoriasis i.e. it tends to run in families. Psoriasis is also influenced by many environmental factors. It is not contagious and is not due to an allergy.

The most common ages for psoriasis to first appear are in the late teens and in the 50s. It affects men and women equally, although in children, girls are more commonly affected than boys.
Psoriasis is often so mild it is barely noticed by the affected person, but it can occasionally so severe the patient must be admitted to hospital for treatment. It may or may not be itchy.
About 5% of those with psoriasis will also develop joint pains (psoriatic arthritis), which may involve one or more joints. This can be very debilitating.

Psoriasis is characterised by red, scaly patches of skin, which usually have very well defined edges. It is often symmetrical, affecting both sides of the body. The scale is typically silvery white. This typical scale may not be so obvious if the psoriasis affects a body fold such as the armpit or the patient is using emollients regularly. Then it is more likely to be smooth and shiny.

Psoriasis is often very minor, causing a few dry patches on the backs of the elbows and knees, some irritation in the umbilicus (belly button) and natal cleft (between the buttocks) or scaling in the scalp. It can, however, affect any area of skin and it may be severe or atypical.

Typical patterns of psoriasis include:

  • Chronic plaque psoriasis: large flat patches (plaques) covered in scale, most often on elbows, knees and lower back
  • Flexural psoriasis: smooth well-defined patches in body folds
  • Scalp psoriasis: one or more scaly plaques in the scalp
  • Acute guttate psoriasis: numerous and often widespread small patches
  • Sebopsoriasis: overlap of seborrhoeic dermatitis and psoriasis, affecting scalp, face, ears and chest
  • Palmoplantar psoriasis: several patterns of psoriasis on the palms and soles
  • Acrodermatitis continua of Hallopeau: pustules affecting the tips of one or more digits and finger and/or toenails
  • Nail psoriasis: pitting, onycholysis, yellowing and ridging
  • Intraoral psoriasis: desquamation inside the mouth, most often associated with the more severe forms of cutaneous psoriasis
  • Koebnerised psoriasis: psoriasis arising in healing wounds or scars
  • Photosensitive psoriasis: psoriasis affecting sun-exposed skin
  • Pustular psoriasis: generalised or localised to palms and soles
  • Erythrodermic psoriasis: severe psoriasis affecting the entire skin surface
  • Psoriatic arthritis: joint disease related to psoriasis

After the psoriasis has cleared up by itself or with treatment, it may leave dark or pale marks (post-inflammatory hyperpigmentation and hypopigmentation). It does not cause true scarring. The pigmentary changes gradually improve over several months.

Exactly what causes psoriasis is not fully understood but there is a lot of active research into this area. The immune system is involved and appears to be overactive in a way that causes inflammation. Specifically, there is excessive production of TH1 cytokines, particularly TNFα. These have many effects, including growth of extra blood vessels within the skin (causing the red colour) and increased turnover of the skin cells (causing the scaling and thickening of the skin).