Why Psoriasis Is an Autoimmune Disease

Psoriasis is an autoimmune disease, meaning that the body's immune system turns on itself and attacks normal cells and tissues—in this case, skin cells known as keratinocytes.

Because psoriasis is caused by an immune system gone awry, it is not contagious like most skin infections. You can neither "catch" it nor "spread" it.

This article explains what autoimmune diseases are and the various immune mechanisms that lead to psoriasis. It also describes the risk factors for this autoimmune skin disease and how it is treated.

Psoriasis and the Immune System

If you have an autoimmune disease, your immune system responds abnormally to a body part, such as the skin, a joint, or the intestine. Psoriasis is one example of at least 28 diseases classified as autoimmune and possibly up to 150 in total, according to the National Institutes of Health.

The immune system is meant to defend the body against foreign invaders. But with psoriasis, the immune system regards skin cells as foreign and mounts an assault in the form of targeted inflammation.

With an autoimmune disease, there are several "players" involved in the abnormal immune response, namely:

• Autoantibodies: Normally, the immune system will produce proteins called antibodies that recognize and "tag" foreign invaders for destruction. With an autoimmune disease, the immune system produces autoantibodies that "tag" normal cells for destruction.
• Antigens: Every antibody is produced to match a protein on the surface of a cell, called an antigen, which it binds to. With an autoimmune disease, the antigen of a normal cell is targeted. Because normal cells are not neutralized in the way that most infections are, the attack will continue unabated, leading to chronic (persistent or recurrent) disease.
• Lymphocytes: These are white blood cells, specifically known as T-cell lymphocytes and B-cell lymphocytes, that are central to the body's immune response. When an autoantibody "tags" a normal cell for destruction, these cells coordinate the immune assault, exposing the "invader" to inflammation to kill it. Inflammation is the body's natural response to all infections and injuries.

Under normal circumstances, skin cells turn over at a steady rate so that old cells can be shed from the surface of the skin (called the epidermis) and replaced with new ones. With psoriasis, the inflammatory autoimmune assault causes skin cells to turn over at a vastly accelerated rate.

When this happens, skin cells will start to pile up on the surface of the skin and become compressed rather than being shed. This causes the red, scaly, itchy patches recognized as plaques. Plaques most commonly develop on the back of the forearms, shins, navel area, and scalp (although any body part can be affected).

Plaque psoriasis is the most common type of psoriasis, accounting for more than 90% of cases. Less common types include pustular psoriasis (which causes pus-filled lesions), guttate psoriasis (which causes drop-shaped lesions), and inverse psoriasis (which affect skin folds).

What Causes Psoriasis?

The cause of psoriasis is unknown. Both genetic and environmental factors are believed to play a role in the onset of the disease.

The theory is genetics, either inherited from your parents or acquired later in life, can predispose a person to the disease. Thereafter, exposure to one or more environmental factors, such as an infection, might "confuse" the immune system and cause it to regard the antigen of a normal cell as hostile.

This can cause not only the onset of psoriasis symptoms but also episodic exacerbations (flare-ups).

Risk factors linked to the onset and/or exacerbation of psoriasis include:

• A family history of psoriasis
• Infections, such as Streptococcus and Candida species
• Air pollution, including exposure to polycyclic aromatic hydrocarbons, ozone, and heavy metals
• Ultraviolet (UV) damage to the skin caused by excess sun exposure
• Certain drugs, like beta-blockers, ACE inhibitors, lithium, and interferon
• Certain vaccines, like the Bacillus Calmette–Guerin (BCG) vaccine and even the flu vaccine
• Lifestyle factors, such as smoking, obesity, and metabolic syndrome
• Mental stress
• Diabetes, especially type 2 diabetes
• Having another autoimmune disease

How Psoriasis Is Treated

Psoriasis treatment is primarily focused on calming the overreactive immune response. At the same time, an effort is made to avoid triggers that cause flare-ups and to minimize symptoms by protecting the skin barrier.

Depending on the severity of symptoms and type of psoriasis involved, the treatment may include some or all of the following:

• Emollient skin creams and lotions to protect the skin barrier
• Topical or oral corticosteroids (steroids) to calm the immune response
• Topical or oral retinoids that slow the production of skin cells
• Topical vitamin D analogs that also slow skin cell production
• Topical calcineurin inhibitors that can reduce the immune overreaction
• Topical coal tar, a centuries-old remedy that slows skin cell turnover
• Phototherapy, a form of light therapy that reduces inflammation and slows skin cell production
• Oral immunosuppressants like methotrexate and Sandimmune (cyclosporine)
• Injected biologic drugs like Otezla (apremilast), Enbrel (etanercept), and Humira (adalimumab) that tone down specific parts of the immune system linked to autoimmunity