The genetic link of psoriasis has long been known; it tends to run in families. About 30% of people with one first degree relative with psoriasis have chances of developing the disease during their life time.
Just having a close relative with psoriasis does not mean that a person could definitely get the disease. This genetic tendency needs to be triggered by some environmental, physiological or psychological factors for a person to get psoriasis.
There is no way of predicting who will develop psoriasis and who will not. 50-60% of people who have psoriasis do not know anyone in their family with the disease.
The exact cause of psoriasis remains a mystery still. As mentioned above, whether a person develops psoriasis or not may depend on a "trigger." Possible psoriasis triggers include emotional stress, skin injury, climatic changes, systemic infections, alcohol, smoking and certain medications.
Psoriasis patients can develop lesions at the site of skin injury involving the deeper layers of the skin, especially during a period of active disease. Psoriasis worsens in areas of skin scrapes, scratches, and cuts (such as surgical wounds). That’s why it is advisable not to pick or scrub the lesions and scales of psoriasis. The development of a psoriatic lesion at the site of skin injury is known as Koebner’s phenomenon.
Psoriasis Skin Changes
It has been suggested that in predisposed individuals, certain epidermal triggering events induce activation of the basal cell (generating cells) layer of the epidermis stimulated by activated T lymphocytes (immune cells) in the blood vessels immediately beneath the epidermis. This results in increased turnover of epidermal cells which reach the top layer within 3-4 days instead of the normal 28 days. These are the silvery white thick scales, the typical feature of psoriasis.
The initial change in psoriasis skin is the dilation of capillaries (small blood vessels in the dermis) and edema of the papillary dermis, which is that part of the dermis immediately below the epidermis. The inflammation causes round white blood cells called mononuclear cells to crowd around these blood vessels. Some of these cells move into the lower epidermal cells causing mild swelling of the cells there. There is also squirting of neutrophils (another type of white blood cells) by the dilated capillaries into the epidermis, which may form micro abscesses within the epidermal layers.
In normal skin, epidermal cells which are formed in the basal layer are pushed up by newly formed cells from beneath. They move towards the superficial layers, gradually maturing and then getting shed from the skin surface in about 28 days. This is an ongoing process and is not very visible in normal skin (except for the dry scales seen in people with dry skin, especially in cold weather).
In people with psoriasis, due to the immunological and cellular events mentioned above, this process is accelerated to 3 to 5 days. This excessive reproduction causes skin cells to build up and form abnormal scaling seen in psoriasis. The scales seen are actually immature or premature epidermal cells, as they do not get time to mature within 3-5 days. The redness beneath is because of the dilated and tortuous capillaries in the papillary dermis, which becomes exposed due to the loosely adherent scales of psoriasis.
Next: Psoriasis Clinical Classification
Reference
Psoriasis, in Rook’s Textbook of Dermatology, 7th Edition, 2004
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