Psoriasis - Causes

Highlights	Diagnosis	Managing Psoriasis
Introduction	Treatment	Resources
Causes	Topical Medications	References
Risk Factors	Other Medications
Prognosis	Phototherapy

Causes

The precise causes of psoriasis are unknown. It is generally believed that psoriasis is a disorder in which factors in the immune system, enzymes, and other biochemical substances that regulate skin cell division become impaired. This abnormal immune response causes rapid proliferation of keratinocytes (immature skin cells) and inflammation. Such events are likely to be triggered by environmental factors, such as weather or stress, in people with genetic factors that make them susceptible.

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Inflammatory Response and Autoimmunity

The Normal Immune System Response. The inflammatory process is a byproduct of the body's immune system, which fights infection and heals wounds and injuries:

When an injury or an infection occurs, white blood cells are mobilized to rid the body of any foreign invaders, such as bacteria or viruses.
The masses of blood cells that gather at the injured or infected site produce factors to repair wounds, clot the blood, and fight any infective agents.
In the process, the surrounding area becomes inflamed and some healthy tissue is injured.
Under normal conditions, the immune system has other factors that control and limit this inflammatory process.

The Infection Fighters. The primary infection-fighting units are two types of white blood cells: lymphocytes and leukocytes.

Lymphocytes include two subtypes known as T cells and B cells. Both types of cells are designed to recognize foreign substances (antigens) and to launch an offensive or defensive action against them:

B cells produce antibodies, which are designed to attack the antigens. Antibodies can either ride along with a B cell or travel on their own.
T cells have special receptors attached to their surface that recognize the specific antigen.

T cells are further categorized as killer T cells or helper T cells (TH cells).

Killer T cells directly attack antigens found on bacteria or other cells.
Helper T cells also recognize antigens, but their role is two fold. They stimulate B cells and other white cells to attack the antigen. They also produce cytokines, powerful immune factors that have an important role in the inflammatory process.

Helper T cells, Cytokines, and the Inflammatory Response. The actions of the helper T cells (TH cells) are of special interest. Researchers have observed high numbers of TH cells in psoriatic plaques:

The activated TH cells infiltrate the skin cells in psoriasis and, in the case of psoriatic arthritis, also the joints. (There has been some debate over whether psoriatic arthritis is a unique disorder, but evidence now suggests that both psoriatic arthritis and psoriasis are caused by the same faulty immune process.)
TH cells normally stimulate B cells to produce antibodies. In the case of psoriasis, however, they appear to direct the B cells to produce autoantibodies ("self" antibodies), which are directed against the body's own cells. In the case of psoriasis, they target self antigens in skin cells; in psoriatic arthritis, cells in the joints also come under attack.
In the resulting autoimmune process, autoantibodies remain in circulation and continue to mount an immune attack against these cells.