In psoriasis, certain immune system cells become overactive and cause the proliferation of skin cells called keratinocytes. Keratinocytes grow in the lower layers of the skin and slowly rise to the surface, a process called turnover that in most people takes about a month but in those with psoriasis takes only a few days (see "Anatomy of psoriasis"). When their turnover is sped up in this way, the keratinocytes rise too quickly and pile up on the surface of the skin, producing thick, scaly red patches, called psoriatic plaques. These patches are red because small blood vessels grow to supply blood and nutrients to the area; these vessels also leak, allowing more immune cells to migrate into the already inflamed skin.
Anatomy of Psoriasis
Psoriasis occurs when skin cells called keratinocytes turn over too rapidly. They form in the lower portion of the epidermis, the outer layer of skin. Normally, it takes about 28 days for these cells to move from the basal (lower) epidermis to the stratum corneum (upper layer). In a person with psoriasis, the process takes only three to four days. The cells quickly pile up and form raised red patches covered with flaking, silvery scales.
The underlying cause of this immune system malfunction is unknown, but the evidence suggests a complex interaction between genetic predisposition (about 40% of people with psoriasis have a family history of the disease) and environmental and lifestyle influences, including obesity, smoking, and heavy drinking. According to one theory, the culprit is an overreaction by the immune system to a triggering incident — for example, stress, an infection (such as strep throat), a skin injury, or a medication (beta blockers or lithium, for example).
Another theory sees psoriasis as an autoimmune condition involving tumor necrosis factor-alpha (TNF-alpha), a substance involved in the body's normal response to infection. It's overproduced in people with psoriasis and may be provoking an immune system attack on their own tissues.
Links with other Conditions
There's increasing evidence that psoriasis, especially severe psoriasis, is associated with several other serious medical conditions. Two 2009 studies suggest that people with psoriasis have a higher rate of atherosclerosis, peripheral artery disease, heart attacks, strokes, and death from cardiovascular disease.
Psoriasis doesn't cause any of these diseases, but they share a couple of features, namely the presence of inflammation and the activity of certain proteins, called cytokines. Inflammation and cytokines are responsible for many of the symptoms of psoriasis and are also involved in the development of artery-clogging atherosclerosis and insulin resistance as well as possibly loss of bone density.
Whether treating psoriasis could reduce your risk of heart disease is unclear. But if you have psoriasis, you should pay especially close attention to cardiovascular risk factors, including blood pressure, cholesterol levels, smoking, exercise, and diet.