This skin layer, located below the epidermis, contains nerves and blood and lymphatic vessels. In psoriasis, these blood vessels provide an increased blood supply to the aberrantly multiplying keratinocytes, causing the underlying inflammation and redness characteristic of psoriasis. Increasingly, it is believed that these destructive changes originate from genetic abnormalities in the immune system that are triggered by environmental factors. A number of psoriasis variants exist. The most common variant is plaque psoriasis, in which the patches start off in small areas, about one-eighth of an inch in diameter, but gradually enlarge and develop plaque.
Patches usually appear symmetrically, that is, in the same areas on opposite sides of the body.
They most often occur on the scalp, elbows, knees, and the lower back. Patches also can appear on the palms and soles, in the genital areas of both men and women, above the pelvic bone, and on the thighs and calves of the legs. At the onset of the disease, the patches are separate, but they may join together to form larger areas as the disorder develops. Patches may persist for long periods, but more often they flare-up periodically, triggered by certain factors, such as cold weather, infection, or stress. Guttate Psoriasis Guttate psoriasis consists of teardrop patches that erupt suddenly, usually over the trunk and often on the arms, legs, or scalp.