Psoriasis occurs when skin cells are replaced more quickly than usual. It is not known exactly why this happens.

Your body produces new cells in the deepest layer of your skin level. These skin cells gradually move up through the layers of skin until they reach the outermost level. Then they die and flake off. This whole process normally takes around three to four weeks.

In people with psoriasis, this process only takes about three to seven days. As a result, cells that are not fully mature build up rapidly on the surface of the skin, causing red, flaky, crusty patches covered with silvery scales.

It's thought that the skin cells are replaced quickly in people with psoriasis due to a problem with the immune system.

Problems with the immune system

Your immune system is your body's defence against disease and it helps fight infection. One of the main types of cell used by the immune system is called a T-cell.

T-cells normally travel through the body to detect and fight things like infections, but in people with psoriasis they start to attack healthy skin cells by mistake. This causes the deepest layer of skin to produce new skin cells more quickly than usual, which in turn triggers the immune system to produce more T-cells.

It is not known what exactly causes this problem with the immune system, although certain genes and environmental triggers may play a role.

Genetics

Psoriasis runs in families. One in three people with psoriasis has a close relative with the condition.

However, the exact role that genetics plays in causing psoriasis is unclear. Research studies have shown  many different genes are linked to the development of psoriasis. It is likely that different combinations of genes may make people more vulnerable to the condition. However, having these genes does not necessarily mean you will develop it.

Psoriasis triggers

Many people's psoriasis symptoms start or become worse because of a certain event, known as a trigger. Knowing your triggers may help you to avoid a flare-up. Common triggers include:

• an injury to your skin such as a cut, scrape, insect bite or sunburn (this is known as the Koebner response)
• drinking excessive amounts of alcohol
• smoking
• stress
• hormonal changes, particularly in women (for example during puberty and the menopause)
• certain medicines such as lithium, some antimalarial medicines, anti-inflammatory medicines including ibuprofen, ACE inhibitors (used to treat high blood pressure) and beta blockers (used to treat congestive heart failure)
• throat infections - in some people, usually children and young adults, a form of psoriasis called guttate psoriasis (which causes smaller pink patches, often without a lot of scaling) develops after a streptococcal throat infection, although most people who have streptococcal throat infections do not develop psoriasis
• other immune disorders, such as HIV, which cause psoriasis to flare up or to appear for the first time

Psoriasis is not contagious so it cannot be spread from person to person.