Psoriasis occurs when skin cells are replaced more quickly than usual. It's not known exactly why this happens, but research suggests it's caused by a problem with the immune system.
Your body produces new skin cells in the deepest layer of skin. 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 aren't fully mature build up rapidly on the surface of the skin, causing red, flaky, crusty patches covered with silvery scales.
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 invading germs such as bacteria, 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's not known what exactly causes this problem with the immune system, although certain genes and environmental triggers may play a role.
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's likely that different combinations of genes may make people more vulnerable to the condition. However, having these genes doesn't necessarily mean you'll develop it.
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
- 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 develops after a streptococcal throat infection (although most people who have streptococcal throat infections don't develop psoriasis)
- other immune disorders, such as HIV, which cause psoriasis to flare up or to appear for the first time
Psoriasis isn't contagious, so it can't be spread from person to person.