Cause
The cause of psoriasis is not fully understood. There are two main hypotheses about the process that occurs in the development of the disease. The first considers psoriasis as primarily a disorder of excessive growth and reproduction of skin cells. The problem is simply seen as a fault of the epidermis and its keratinocytes. The second hypothesis sees the disease as being an immune-mediated disorder in which the excessive reproduction of skin cells is secondary to factors produced by the immune system. T cells (which normally help protect the body against infection) become active, migrate to the dermis and trigger the release of cytokines (tumor necrosis factor-alpha TNFα, in particular) which cause inflammation and the rapid production of skin cells. It is not known what initiates the activation of the T cells.
The immune-mediated model of psoriasis has been supported by the observation that immunosuppressant medications can clear psoriasis plaques. However, the role of the immune system is not fully understood, and it has recently been reported that an animal model of psoriasis can be triggered in mice lacking T cells. Animal models, however, reveal only a few aspects resembling human psoriasis.
Compromised skin barrier function has a role in psoriasis susceptibility.
Psoriasis is a fairly idiosyncratic disease. The majority of people's experience of psoriasis is one in which it may worsen or improve for no apparent reason. Studies of the factors associated with psoriasis tend to be based on small (usually hospital based) samples of individuals. These studies tend to suffer from representative issues, and an inability to tease out causal associations in the face of other (possibly unknown) intervening factors. Conflicting findings are often reported. Nevertheless, the first outbreak is sometimes reported following stress (physical and mental), skin injury, and streptococcal infection. Conditions that have been reported as accompanying a worsening of the disease include infections, stress, and changes in season and climate. Certain medicines, including lithium salt, beta blockers and the antimalarial drug chloroquine have been reported to trigger or aggravate the disease. Excessive alcohol consumption, smoking and obesity may exacerbate psoriasis or make the management of the condition difficult or perhaps these comorbidities are effects rather than causes. Hairspray, some face creams and hand lotions, can also cause an outbreak of psoriasis. In 1975, Stefania Jablonska and collaborators advanced a new theory that special antibodies tend to break through into the lower layers of the skin and set up a complex series of chemical reactions.
Individuals suffering from the advanced effects of the human immunodeficiency virus, or HIV, often exhibit psoriasis. This presents a paradox to researchers, as traditional therapies that reduce T-cell counts generally cause psoriasis to improve. Yet, as CD4-T-cell counts decrease with the progression of HIV, psoriasis worsens. In addition, HIV is typically characterized by a strong Th2 cytokine profile, whereas psoriasis vulgaris is characterized by a strong Th1 secretion pattern. It is hypothesized that the diminished CD4-T-Cell presence causes an overactivation of CD8-T-cells, which are responsible for the exacerbation of psoriasis in HIV positive patients. It is important to remember that most individuals with psoriasis are otherwise healthy, and the presence of HIV accounts for less than 1% of cases. The prevalence of psoriasis in the HIV positive population ranges from 1 to 6 percent, which is about three times higher than the normal population. Psoriasis in AIDS sufferers is often severe, and untreatable with conventional therapy.
Psoriasis occurs more likely in dry skin than oily or well-moisturized skin, and specifically after an external skin injury such as a scratch or cut (see Koebner phenomenon). This is believed to be caused by an infection, in which the infecting organism thrives under dry skin conditions with minimal skin oil, which otherwise protects skin from infections. The case for psoriasis is opposite to the case of athlete's foot, which occurs because of a fungus infection under wet conditions as opposed to dry in psoriasis. This infection induces inflammation, which causes the symptoms commonly associated with psoriasis, such as itching and rapid skin turnover, and leads to drier skin, as the infecting organism absorbs the moisture that would otherwise go to the skin. To prevent dry skin and reduce psoriasis symptoms, it is advised to not use shower scrubs, as they not only damage skin by leaving tiny scratches, but they also scrape off the naturally occurring skin oil. Additionally, moisturizers can be applied to moisturize the skin, and lotions used to promote skin oil gland functions.
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