National Psoriasis Foundation

The Immune System and Psoriatic Disease

What is an autoimmune disease?

Researchers agree that psoriatic disease is an autoimmune disease. That means that psoriasis and psoriatic arthritis are actually caused by an overactive immune system. But how can your immune system—which is built to keep you healthy—actually cause an illness? The explanation can be found in the word itself. Autoimmunity occurs when the immune system automatically launches an inflammatory response against your own body.

When the immune system functions properly, it protects the body against any “invaders” that might make you sick, such as bacteria, viruses or other pathogens. But in people with psoriasis and psoriatic arthritis, the immune system goes into action even without these invaders. Instead, the immune system fights the body’s own tissues. In psoriatic disease, this battle is waged in the skin and joints.

Researchers who study psoriatic disease are still working to identify the substances inside the body that the immune response mistakes for antigens. One possibility could be certain kinds of bacteria. For example, in some cases, streptococcal infection (known as strep throat) can trigger a case of guttate psoriasis. Another possible antigen could be antimicrobial peptides, molecules that are a part of the immune system and work as the body’s own antibiotics. Research funded by the National Psoriasis Foundation found that a particular antimicrobial peptide can cause an autoimmune reaction in many people with moderate to severe psoriasis.

The role of inflammation

Inflammation is one of the weapons used by the immune system to fight an invader. For example, when you catch a virus or develop a bacterial infection, a type of immune cell called a T cell springs into action. When T cells recognize something as an invader – also called an antigen—T cells begin an inflammatory attack against the invader.

This attack is carried out by cytokines, which are proteins that help control the immune system’s inflammatory response. Cytokines trigger inflammation, causing the blood vessels to expand and send more immune cells to different parts of the body. In psoriasis, this inflammation happens in the skin, leading to the red, itchy and scaly patches known as plaques. In psoriatic arthritis, this inflammation happens inside the body, leading to swollen and painful joints and tendons.

Treating the immune system

The immune system is not only the key to what causes psoriatic disease—it may be the key to treating it, too. In 1979, researchers discovered on accident that a drug called cyclosporine that suppresses the immune system also clears psoriasis. This offered one of the first clues that psoriasis was actually an autoimmune disease. Since then, many effective treatments directed toward the immune system have been developed for psoriasis and psoriatic arthritis.

Methotrexate, another systemic drug that suppresses the immune system, is often used to treat psoriasis and psoriatic arthritis. Newer drugs called biologics affect certain parts of the immune system to treat psoriatic disease.

What's next?

Scientists are continuing to study the complex relationship between the immune system and psoriatic disease. Researchers are working to identify the antigens that trigger the autoimmune response in psoriasis and psoriatic arthritis, to better understand the role played by different kinds of immune cells in psoriatic disease, and develop new therapies that target cytokines or other parts of the immune system.

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